fix SNTP through proper protocol handler setup

src/sntp.c

@@ -82,12 +82,7 @@ void mg_sntp_request(struct mg_connection *c) {
 }
 
 struct mg_connection *mg_sntp_connect(struct mg_mgr *mgr, const char *url,
-                                      mg_event_handler_t fn, void *fnd) {
-  struct mg_connection *c = NULL;
+                                      mg_event_handler_t fn, void *fn_data) {
   if (url == NULL) url = "udp://time.google.com:123";
-  if ((c = mg_connect(mgr, url, fn, fnd)) != NULL) {
-    c->pfn = sntp_cb;
-    sntp_cb(c, MG_EV_OPEN, (void *) url);
-  }
-  return c;
+  return mg_connect_svc(mgr, url, fn, fn_data, sntp_cb, NULL);
 }