raop: Add autoreconnect feature

This patch adds the autoreconnect feature to the raop module.
This is mainly to be used in a server context, but can be used
also in a desktop usage context.

With autoreconnect feature, the raop module behaves like this:

- At initialisation or in case of the RTSP TCP connection lost, it
  tries to reconnect every 5 seconds
- In case of any fatal error, it tries to reconnect every 5 seconds
- In UDP mode, if no timing packets received anymore for a long time,
  RTSP connection is closed, then it tries to reconnect..
- After reconnection, once RTSP session has been established again,
  playing is resumed automatically.
- When the connection is not established yet (or loss), the sink
  behaves like a null sink. In the source code I called it "autonull",
  even if autonull is set to autoreconnect param value, it could be
  split into two different params.

src/modules/raop/module-raop-sink.c

@@ -62,6 +62,7 @@ static const char* const valid_modargs[] = {
     "username",
     "password",
     "latency_msec",
+    "autoreconnect",
     NULL
 };