Add mbedtls_ssl_get_max_frag_len()

This is not very useful for TLS as mbedtls_ssl_write() will automatically
fragment and return the length used, and the application should check for that
anyway, but this is useful for DTLS where mbedtls_ssl_write() returns an
error, and the application needs to be able to query the maximum length
instead of just guessing.

programs/ssl/ssl_server2.c

@@ -1946,6 +1946,11 @@ reset:
     else
         mbedtls_printf( "    [ Record expansion is unknown (compression) ]\n" );
 
+#if defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH)
+    mbedtls_printf( "    [ Maximum fragment length is %u ]\n",
+                    (unsigned int) mbedtls_ssl_get_max_frag_len( &ssl ) );
+#endif
+
 #if defined(MBEDTLS_SSL_ALPN)
     if( opt.alpn_string != NULL )
     {