Use MG_EV_WS_CTL for WS control messages

src/event.h

@@ -29,6 +29,7 @@ enum {
   MG_EV_HTTP_MSG,   // HTTP request/response        struct mg_http_message *
   MG_EV_WS_OPEN,    // Websocket handshake done     NULL
   MG_EV_WS_MSG,     // Websocket message received   struct mg_ws_message *
+  MG_EV_WS_CTL,     // Websocket control message    struct mg_ws_message *
   MG_EV_MQTT_CMD,   // MQTT low-level command       struct mg_mqtt_message *
   MG_EV_MQTT_MSG,   // MQTT PUBLISH received        struct mg_mqtt_message *
   MG_EV_MQTT_OPEN,  // MQTT CONNACK received        int *connack_status_code