Automatically allocate opaque PSK key slots in ssl_{client/server}2

Previously, command line arguments `psk_slot` and `psk_list_slot`
could be used to indicate the PSA key slots that the example
applications should use to store the PSK(s) provided.

This commit changes this approach to use the utility function
`mbedtls_psa_get_free_key_slot()` to obtain free key slots from
the PSA Crypto implementation automatically, so that users only
need to pass boolean flags `psk_opaque` and `psk_list_opaque`
on the command line to enable / disable PSA-based opaque PSKs.
diff --git a/tests/ssl-opt.sh b/tests/ssl-opt.sh
index 45b2e20..129b91c 100755
--- a/tests/ssl-opt.sh
+++ b/tests/ssl-opt.sh
@@ -3834,7 +3834,7 @@
 run_test    "PSK callback: opaque psk on client, no callback" \
             "$P_SRV extended_ms=0 debug_level=1 psk=abc123 psk_identity=foo" \
             "$P_CLI extended_ms=0 debug_level=1 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-128-CBC-SHA \
-            psk_identity=foo psk=abc123 psk_slot=1" \
+            psk_identity=foo psk=abc123 psk_opaque=1" \
             0 \
             -c "skip PMS generation for opaque PSK"\
             -S "skip PMS generation for opaque PSK"\
@@ -3848,7 +3848,7 @@
 run_test    "PSK callback: opaque psk on client, no callback, SHA-384" \
             "$P_SRV extended_ms=0 debug_level=1 psk=abc123 psk_identity=foo" \
             "$P_CLI extended_ms=0 debug_level=1 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-256-CBC-SHA384 \
-            psk_identity=foo psk=abc123 psk_slot=1" \
+            psk_identity=foo psk=abc123 psk_opaque=1" \
             0 \
             -c "skip PMS generation for opaque PSK"\
             -S "skip PMS generation for opaque PSK"\
@@ -3862,7 +3862,7 @@
 run_test    "PSK callback: opaque psk on client, no callback, EMS" \
             "$P_SRV extended_ms=1 debug_level=3 psk=abc123 psk_identity=foo" \
             "$P_CLI extended_ms=1 debug_level=3 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-128-CBC-SHA \
-            psk_identity=foo psk=abc123 psk_slot=1" \
+            psk_identity=foo psk=abc123 psk_opaque=1" \
             0 \
             -c "skip PMS generation for opaque PSK"\
             -S "skip PMS generation for opaque PSK"\
@@ -3876,7 +3876,7 @@
 run_test    "PSK callback: opaque psk on client, no callback, SHA-384, EMS" \
             "$P_SRV extended_ms=1 debug_level=3 psk=abc123 psk_identity=foo" \
             "$P_CLI extended_ms=1 debug_level=3 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-256-CBC-SHA384 \
-            psk_identity=foo psk=abc123 psk_slot=1" \
+            psk_identity=foo psk=abc123 psk_opaque=1" \
             0 \
             -c "skip PMS generation for opaque PSK"\
             -S "skip PMS generation for opaque PSK"\
@@ -3888,7 +3888,7 @@
 
 requires_config_enabled MBEDTLS_USE_PSA_CRYPTO
 run_test    "PSK callback: raw psk on client, static opaque on server, no callback" \
-            "$P_SRV extended_ms=0 debug_level=1 psk=abc123 psk_identity=foo psk_slot=1 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-128-CBC-SHA" \
+            "$P_SRV extended_ms=0 debug_level=1 psk=abc123 psk_identity=foo psk_opaque=1 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-128-CBC-SHA" \
             "$P_CLI extended_ms=0 debug_level=1 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-128-CBC-SHA \
             psk_identity=foo psk=abc123" \
             0 \
@@ -3902,7 +3902,7 @@
 
 requires_config_enabled MBEDTLS_USE_PSA_CRYPTO
 run_test    "PSK callback: raw psk on client, static opaque on server, no callback, SHA-384" \
-            "$P_SRV extended_ms=0 debug_level=1 psk=abc123 psk_identity=foo psk_slot=1 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-256-CBC-SHA384" \
+            "$P_SRV extended_ms=0 debug_level=1 psk=abc123 psk_identity=foo psk_opaque=1 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-256-CBC-SHA384" \
             "$P_CLI extended_ms=0 debug_level=1 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-256-CBC-SHA384 \
             psk_identity=foo psk=abc123" \
             0 \
@@ -3916,7 +3916,7 @@
 
 requires_config_enabled MBEDTLS_USE_PSA_CRYPTO
 run_test    "PSK callback: raw psk on client, static opaque on server, no callback, EMS" \
-            "$P_SRV debug_level=3 psk=abc123 psk_identity=foo psk_slot=1 min_version=tls1_2 \
+            "$P_SRV debug_level=3 psk=abc123 psk_identity=foo psk_opaque=1 min_version=tls1_2 \
             force_ciphersuite=TLS-PSK-WITH-AES-128-CBC-SHA extended_ms=1" \
             "$P_CLI debug_level=3 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-128-CBC-SHA \
             psk_identity=foo psk=abc123 extended_ms=1" \
@@ -3931,7 +3931,7 @@
 
 requires_config_enabled MBEDTLS_USE_PSA_CRYPTO
 run_test    "PSK callback: raw psk on client, static opaque on server, no callback, EMS, SHA384" \
-            "$P_SRV debug_level=3 psk=abc123 psk_identity=foo psk_slot=1 min_version=tls1_2 \
+            "$P_SRV debug_level=3 psk=abc123 psk_identity=foo psk_opaque=1 min_version=tls1_2 \
             force_ciphersuite=TLS-PSK-WITH-AES-256-CBC-SHA384 extended_ms=1" \
             "$P_CLI debug_level=3 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-256-CBC-SHA384 \
             psk_identity=foo psk=abc123 extended_ms=1" \
@@ -3946,7 +3946,7 @@
 
 requires_config_enabled MBEDTLS_USE_PSA_CRYPTO
 run_test    "PSK callback: raw psk on client, no static PSK on server, opaque PSK from callback" \
-            "$P_SRV extended_ms=0 debug_level=3 psk_list=abc,dead,def,beef psk_list_slot=1 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-128-CBC-SHA" \
+            "$P_SRV extended_ms=0 debug_level=3 psk_list=abc,dead,def,beef psk_list_opaque=1 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-128-CBC-SHA" \
             "$P_CLI extended_ms=0 debug_level=3 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-128-CBC-SHA \
             psk_identity=def psk=beef" \
             0 \
@@ -3960,7 +3960,7 @@
 
 requires_config_enabled MBEDTLS_USE_PSA_CRYPTO
 run_test    "PSK callback: raw psk on client, no static PSK on server, opaque PSK from callback, SHA-384" \
-            "$P_SRV extended_ms=0 debug_level=3 psk_list=abc,dead,def,beef psk_list_slot=1 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-256-CBC-SHA384" \
+            "$P_SRV extended_ms=0 debug_level=3 psk_list=abc,dead,def,beef psk_list_opaque=1 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-256-CBC-SHA384" \
             "$P_CLI extended_ms=0 debug_level=3 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-256-CBC-SHA384 \
             psk_identity=def psk=beef" \
             0 \
@@ -3974,7 +3974,7 @@
 
 requires_config_enabled MBEDTLS_USE_PSA_CRYPTO
 run_test    "PSK callback: raw psk on client, no static PSK on server, opaque PSK from callback, EMS" \
-            "$P_SRV debug_level=3 psk_list=abc,dead,def,beef psk_list_slot=1 min_version=tls1_2 \
+            "$P_SRV debug_level=3 psk_list=abc,dead,def,beef psk_list_opaque=1 min_version=tls1_2 \
             force_ciphersuite=TLS-PSK-WITH-AES-128-CBC-SHA extended_ms=1" \
             "$P_CLI debug_level=3 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-128-CBC-SHA \
             psk_identity=abc psk=dead extended_ms=1" \
@@ -3989,7 +3989,7 @@
 
 requires_config_enabled MBEDTLS_USE_PSA_CRYPTO
 run_test    "PSK callback: raw psk on client, no static PSK on server, opaque PSK from callback, EMS, SHA384" \
-            "$P_SRV debug_level=3 psk_list=abc,dead,def,beef psk_list_slot=1 min_version=tls1_2 \
+            "$P_SRV debug_level=3 psk_list=abc,dead,def,beef psk_list_opaque=1 min_version=tls1_2 \
             force_ciphersuite=TLS-PSK-WITH-AES-256-CBC-SHA384 extended_ms=1" \
             "$P_CLI debug_level=3 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-256-CBC-SHA384 \
             psk_identity=abc psk=dead extended_ms=1" \
@@ -4004,7 +4004,7 @@
 
 requires_config_enabled MBEDTLS_USE_PSA_CRYPTO
 run_test    "PSK callback: raw psk on client, mismatching static raw PSK on server, opaque PSK from callback" \
-            "$P_SRV extended_ms=0 psk_identity=foo psk=abc123 debug_level=3 psk_list=abc,dead,def,beef psk_list_slot=1 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-128-CBC-SHA" \
+            "$P_SRV extended_ms=0 psk_identity=foo psk=abc123 debug_level=3 psk_list=abc,dead,def,beef psk_list_opaque=1 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-128-CBC-SHA" \
             "$P_CLI extended_ms=0 debug_level=3 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-128-CBC-SHA \
             psk_identity=def psk=beef" \
             0 \
@@ -4018,7 +4018,7 @@
 
 requires_config_enabled MBEDTLS_USE_PSA_CRYPTO
 run_test    "PSK callback: raw psk on client, mismatching static opaque PSK on server, opaque PSK from callback" \
-            "$P_SRV extended_ms=0 psk_slot=1 psk_identity=foo psk=abc123 debug_level=3 psk_list=abc,dead,def,beef psk_list_slot=2 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-128-CBC-SHA" \
+            "$P_SRV extended_ms=0 psk_opaque=1 psk_identity=foo psk=abc123 debug_level=3 psk_list=abc,dead,def,beef psk_list_opaque=1 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-128-CBC-SHA" \
             "$P_CLI extended_ms=0 debug_level=3 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-128-CBC-SHA \
             psk_identity=def psk=beef" \
             0 \
@@ -4032,7 +4032,7 @@
 
 requires_config_enabled MBEDTLS_USE_PSA_CRYPTO
 run_test    "PSK callback: raw psk on client, mismatching static opaque PSK on server, raw PSK from callback" \
-            "$P_SRV extended_ms=0 psk_slot=1 psk_identity=foo psk=abc123 debug_level=3 psk_list=abc,dead,def,beef min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-128-CBC-SHA" \
+            "$P_SRV extended_ms=0 psk_opaque=1 psk_identity=foo psk=abc123 debug_level=3 psk_list=abc,dead,def,beef min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-128-CBC-SHA" \
             "$P_CLI extended_ms=0 debug_level=3 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-128-CBC-SHA \
             psk_identity=def psk=beef" \
             0 \
@@ -4045,7 +4045,7 @@
 
 requires_config_enabled MBEDTLS_USE_PSA_CRYPTO
 run_test    "PSK callback: raw psk on client, id-matching but wrong raw PSK on server, opaque PSK from callback" \
-            "$P_SRV extended_ms=0 psk_slot=1 psk_identity=def psk=abc123 debug_level=3 psk_list=abc,dead,def,beef min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-128-CBC-SHA" \
+            "$P_SRV extended_ms=0 psk_opaque=1 psk_identity=def psk=abc123 debug_level=3 psk_list=abc,dead,def,beef min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-128-CBC-SHA" \
             "$P_CLI extended_ms=0 debug_level=3 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-128-CBC-SHA \
             psk_identity=def psk=beef" \
             0 \
@@ -4058,7 +4058,7 @@
 
 requires_config_enabled MBEDTLS_USE_PSA_CRYPTO
 run_test    "PSK callback: raw psk on client, matching opaque PSK on server, wrong opaque PSK from callback" \
-            "$P_SRV extended_ms=0 psk_slot=1 psk_identity=def psk=beef debug_level=3 psk_list=abc,dead,def,abc123 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-128-CBC-SHA" \
+            "$P_SRV extended_ms=0 psk_opaque=1 psk_identity=def psk=beef debug_level=3 psk_list=abc,dead,def,abc123 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-128-CBC-SHA" \
             "$P_CLI extended_ms=0 debug_level=3 min_version=tls1_2 force_ciphersuite=TLS-PSK-WITH-AES-128-CBC-SHA \
             psk_identity=def psk=beef" \
             1 \