Rename psa_internal_allocate_key_slot to psa_get_empty_key_slot

This function no longer modifies anything, so it doesn't actually
allocate the slot. Now, it just returns the empty key slot, and it's
up to the caller to cause the slot to be in use (or not).
diff --git a/library/psa_crypto_slot_management.c b/library/psa_crypto_slot_management.c
index 0734009..fe92148 100644
--- a/library/psa_crypto_slot_management.c
+++ b/library/psa_crypto_slot_management.c
@@ -102,7 +102,7 @@
     global_data.key_slots_initialized = 0;
 }
 
-psa_status_t psa_internal_allocate_key_slot( psa_key_handle_t *handle,
+psa_status_t psa_get_empty_key_slot( psa_key_handle_t *handle,
                                              psa_key_slot_t **p_slot )
 {
     if( ! global_data.key_slots_initialized )
@@ -228,7 +228,7 @@
     if( status != PSA_SUCCESS )
         return( status );
 
-    status = psa_internal_allocate_key_slot( handle, &slot );
+    status = psa_get_empty_key_slot( handle, &slot );
     if( status != PSA_SUCCESS )
         return( status );