Add WebRTC support using libdatachannel

The WebRTC is exposed via `/video.html` endpoint
and enabled by default as long as h264 stream
is available.

output/webrtc/webrtc.h

@@ -0,0 +1,11 @@
+#pragma once
+
+#include "util/http/http.h"
+#include "util/opts/log.h"
+#include "util/opts/fourcc.h"
+#include "util/opts/control.h"
+#include "device/buffer.h"
+
+// WebRTC
+void http_webrtc_offer(http_worker_t *worker, FILE *stream);
+void webrtc_server();