[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).