[cmyth] Connect Playback or Monitor

- New functions to connect as Monitor or Playback to the backend
[mythtv-cmyth] Connect Playback or Monitor

Add 'playback' flag to the initializer of MythConnection and select annoucement type.
- Recording stream control connection use Playback annoucement to prevent backend shutdown.
- Addon control connection use Monitor annoucement to allow backend shutdown.