[cmyth] Add cmyth_recorder_cancel_next_recording
[mythtv-cmyth] Add Live TV conflict handling

Scheduled recordings will hang in MythTV if ASK_RECORDING is ignored.
Correct responses are either CANCEL_NEXT_RECORDING or stopping playback.

This adds a setting letting the user define how to handle conflicts:
 - Always prefer Live TV
 - Always prefer recording
 - Prefer Live TV only if the recording has later slot (default)

See MythTV: libs/libmythtv/tv_play.cpp: TV::AskAllowRecording.