Clean up RPC response.

- Refactor mqtt_publish_stat() to take va_list
- Make publish_stat format its response with json_vprintf() so it is
  valid JSON.
- Call publish_stat in channel_set, the only place where the GPIO state
  changes
- Return (valid) JSON response in all RPCs

include/mqtt.h

@@ -9,6 +9,6 @@
 #define MQTT_TOPIC_BROADCAST_STAT "/mongoose/broadcast/stat"
 
 void mqtt_init();
-void mqtt_publish_stat(const char *stat, const char *msg);
+void mqtt_publish_stat(const char *stat, const char *fmt, ...);
 
 #endif // __MQTT_H