Make 'port' a string in NET module

- avoids dependency on snprintf
- allows using "smtps" instead of "456" if desired
diff --git a/programs/ssl/ssl_pthread_server.c b/programs/ssl/ssl_pthread_server.c
index 3af66d0..3a137e3 100644
--- a/programs/ssl/ssl_pthread_server.c
+++ b/programs/ssl/ssl_pthread_server.c
@@ -448,7 +448,7 @@
     mbedtls_printf( "  . Bind on https://localhost:4433/ ..." );
     fflush( stdout );
 
-    if( ( ret = mbedtls_net_bind( &listen_fd, NULL, 4433, MBEDTLS_NET_PROTO_TCP ) ) != 0 )
+    if( ( ret = mbedtls_net_bind( &listen_fd, NULL, "4433", MBEDTLS_NET_PROTO_TCP ) ) != 0 )
     {
         mbedtls_printf( " failed\n  ! mbedtls_net_bind returned %d\n\n", ret );
         goto exit;