[cmyth] Cosmetics

- Function cmyth_timestamp_from_tm() is local. Fix storage type as static.
- Function cmyth_timstamp_diff() should return number of seconds. Use difftime() from standard lib.
[mythtv-cmyth] Cosmetics around time_t type

- time_t is undefined arithmetic type. In GetEPGForChannel(), convert startTime to number of minutes since epoch before cast.
- Check times of PVRTimer using time(). If original values equal 0 (NULL), time(NULL) will return current time as expected.
- Fix timeslot for all.