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/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index f9b9502..daa9d74 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -923,7 +923,9 @@
 #if defined(MBEDTLS_HAVE_TIME)
     mbedtls_time_t start;       /*!< starting time      */
 #endif
+#if !defined(MBEDTLS_SSL_SINGLE_CIPHERSUITE)
     int ciphersuite;            /*!< chosen ciphersuite */
+#endif /* MBEDTLS_SSL_SINGLE_CIPHERSUITE */
     int compression;            /*!< chosen compression */
     size_t id_len;              /*!< session id length  */
     unsigned char id[32];       /*!< session identifier */
diff --git a/include/mbedtls/ssl_ciphersuites.h b/include/mbedtls/ssl_ciphersuites.h
index 2f31cee..c8cfacd 100644
--- a/include/mbedtls/ssl_ciphersuites.h
+++ b/include/mbedtls/ssl_ciphersuites.h
@@ -681,6 +681,21 @@
     }
 }
 
+#if !defined(MBEDTLS_SSL_SINGLE_CIPHERSUITE)
+static inline int mbedtls_ssl_session_get_ciphersuite(
+    mbedtls_ssl_session const * session )
+{
+    return( session->ciphersuite );
+}
+#else /* !MBEDTLS_SSL_SINGLE_CIPHERSUITE */
+static inline int mbedtls_ssl_session_get_ciphersuite(
+    mbedtls_ssl_session const * session )
+{
+    ((void) session);
+    return( MBEDTLS_SSL_SUITE_ID( MBEDTLS_SSL_SINGLE_CIPHERSUITE ) );
+}
+#endif /* MBEDTLS_SSL_SINGLE_CIPHERSUITE */
+
 const int *mbedtls_ssl_list_ciphersuites( void );
 
 mbedtls_ssl_ciphersuite_handle_t mbedtls_ssl_ciphersuite_from_string( const char *ciphersuite_name );