Move mg_listen() to net.c

test/mongoose_custom.c

@@ -9,10 +9,9 @@ void mg_connect_resolved(struct mg_connection *c) {
   (void) c;
 }
 
-struct mg_connection *mg_listen(struct mg_mgr *mgr, const char *url,
-                                mg_event_handler_t fn, void *fn_data) {
-  (void) mgr, (void) url, (void) fn, (void) fn_data;
-  return NULL;
+bool mg_open_listener(struct mg_connection *c, const char *url) {
+  (void) c, (void) url;
+  return false;
 }
 
 void mg_mgr_poll(struct mg_mgr *mgr, int ms) {