[mythtv-cmyth] EDL: Use position/duration/seeking since MythTV 0.27

Since MythTV 0.27, the backend provides accurate position/duration/seeking with
non-constant frame rates.

Depending of the backend database version, we check if the new type of mark (33)
exists else fallback using old method based on framerate.

Note that to get the benefit of these changes for preexisting recordings,
it may be necessary to run "mythcommflag --rebuild" on affected recordings.
[mythtv-cmyth] When EDL start_mark is zero, assume 0ms offset. Skip DB seek

mythcommflag does not always rebuild the recordedseek table starting from 0,
which can put the start of a break that is at the beginning of a recording
off by a half-second or more.

Also add in a check to see if a DB seek for end_mark offsets failed. If they
failed, fallback to the static framerate guess.