Make query API for state of MFL extension internal

This commit makes the API

- mbedtls_ssl_get_output_max_frag_len()
- mbedtls_ssl_get_input_max_frag_len()
- mbedtls_ssl_get__max_frag_len()

for querying the state of the Maximum Fragment Length
extension internal.

Rationale: The value those APIs provide to the user is in
upper bounds for the size of incoming and outgoing records,
which can be used to size application data buffers apporpriately
before passing them to mbedtls_ssl_{read,write}(). However,
there are other factors which influence such upper bounds,
such as the MTU or other extensions (specifically, the
record_size_limit extension which is still to be implemented)
which should be taken into account.

There should be more general APIs for querying the maximum
size of incoming and outgoing records.

For the maximum size of outgoing records, we already have such,
namely mbedtls_ssl_get_max_out_record_payload().

For the maximum size of incoming records, a new API will be
added in a subsequent commit.

Signed-off-by: Hanno Becker <hanno.becker@arm.com>

programs/ssl/ssl_client2.c

@@ -2083,13 +2083,6 @@ int main( int argc, char *argv[] )
     else
         mbedtls_printf( "    [ Record expansion is unknown ]\n" );
 
-#if defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH)
-    mbedtls_printf( "    [ Maximum input fragment length is %u ]\n",
-                    (unsigned int) mbedtls_ssl_get_input_max_frag_len( &ssl ) );
-    mbedtls_printf( "    [ Maximum output fragment length is %u ]\n",
-                    (unsigned int) mbedtls_ssl_get_output_max_frag_len( &ssl ) );
-#endif
-
 #if defined(MBEDTLS_SSL_ALPN)
     if( opt.alpn_string != NULL )
     {