[cmyth] Add cmyth_timestamp_utc_from_unixtime
[cmyth] Update cmyth_proginfo_delete_recording

This updates cmyth_proginfo_delete_recording to use the different
versions of DELETE_RECORDING.

 1-55: DELETE RECORDING[]:[]<proginfo>
56-58: DELETE RECORDING chanid recstartts [FORCE/NO_FORCE]
59-  : DELETE RECORDING chanid recstartts [FORCE/NO_FORCE [FORGET/NO_FORGET]]

For newer protocol versions, this allows also to force deletion and
allows to rerecord a recording.
[mythtv-cmyth] Update DeleteRecording and add DeleteAndForgetRecording