[mythtv-cmyth] Start development v1.6.13
[cmyth] Return distinct channel rows

The current version of the query can return duplicated rows. It aggregates
the data for distinct values of service_id.
[mythtv-cmyth] Provide correct ids for merged channels

Channels are merged by the identifier (Number, CallSign). XBMC PVR therefore
only knows about the merged channels.

This adapts the timers and channel group members to always contain the channel
id of the merged channel.

Adds function FindPRVChannelUid() to return the merged channel id for channel.