[cmyth] Review of "GET_RECORDING TIMESLOT"

- Review error handling of cmyth_timestamp functions.
- Add new function cmyth_timestamp_to_numstring() to support DateTime big endian format used by "GET_RECORDING TIMESLOT".
- Remove unix time conversion in cmyth_proginfo_get_from_timeslot() and use cmyth_timestamp_t instead.
[mythtv-cmyth] Review of "GET_RECORDING TIMESLOT"

- Remove unix time conversion and use MythTimestamp class instead.