[cmyth] Handle MythTV priorities correctly

MythTV priorities can be significantly larger than the limits of a int8.
This change allows users with recordings that have priorities outside of the
int8 range to load recordings and use the MythTV PVR addon.

Without this, the addon will fail to populate the recordings list if there are
any recordings that had a priority outside the int8 range.
[cmyth] Update SET_BOOKMARK for protocol >= 77

For protocol >= 77, do not send trailing blank parameter with SET_BOOKMARK
This useless parameter was removed in MythTV 0.27, protocol 77.

Reference:
http://code.mythtv.org/trac/ticket/11104
http://code.mythtv.org/trac/changeset/a8722faea68e8f7d2bb12f31bf78d6bc272dbdde/mythtv
[cmyth] Add function cmyth_mysql_get_recording_seek_offset

Function returns offset for mark and type from position map of recording
(table recordedseek).
[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.