Remove ciphersuite from SSL session if single suite hardcoded

If MBEDTLS_SSL_SINGLE_CIPHERSUITE is enabled, the type

  mbedtls_ssl_ciphersuite_handle_t

is logically a boolean (concretely realized as `unsigned char`),
containing the invalid handle and the unique valid handle, which
represents the single enabled ciphersuite.

The SSL session structure mbedtls_ssl_session contains an instance
of mbedtls_ssl_ciphersuite_handle_t which is guaranteed to be valid,
and which is hence redundant in any two-valued implementation of
mbedtls_ssl_ciphersuite_handle_t.

This commit replaces read-uses of

  mbedtls_ssl_session::ciphersuite_info

by a getter functions which, and defines this getter function
either by just reading the field from the session structure
(in case MBEDTLS_SSL_SINGLE_CIPHERSUITE is disabled), or by
returning the single valid ciphersuite handle (in case
MBEDTLS_SSL_SINGLE_CIPHERSUITE is enabled) and removing the
field from mbedtls_ssl_session in this case.
diff --git a/library/ssl_cli.c b/library/ssl_cli.c
index 498bb79..f4d51dc 100644
--- a/library/ssl_cli.c
+++ b/library/ssl_cli.c
@@ -1845,7 +1845,7 @@
 #if !defined(MBEDTLS_SSL_NO_SESSION_RESUMPTION)
     if( n == 0 ||
         mbedtls_ssl_get_renego_status( ssl ) != MBEDTLS_SSL_INITIAL_HANDSHAKE ||
-        ssl->session_negotiate->ciphersuite != i ||
+        mbedtls_ssl_session_get_ciphersuite( ssl->session_negotiate ) != i ||
         ssl->session_negotiate->compression != comp ||
         ssl->session_negotiate->id_len != n ||
         memcmp( ssl->session_negotiate->id, buf + 35, n ) != 0 )
@@ -1874,7 +1874,9 @@
 #if defined(MBEDTLS_HAVE_TIME)
         ssl->session_negotiate->start = mbedtls_time( NULL );
 #endif
+#if !defined(MBEDTLS_SSL_SINGLE_CIPHERSUITE)
         ssl->session_negotiate->ciphersuite = i;
+#endif /* MBEDTLS_SSL_SINGLE_CIPHERSUITE */
         ssl->session_negotiate->compression = comp;
         ssl->session_negotiate->id_len = n;
         memcpy( ssl->session_negotiate->id, buf + 35, n );