musiconhold: Add preferchannelclass=no option to prefer app class.

The new option 'preferchannelclass' is added to musiconhold.conf. If yes
(the default) the CHANNEL(musicclass) is preferred when choosing the
hold music. If it is no, the class suggested by the application that
calls the MoH (e.g. the Queue() app) gets preferred (new behaviour).

This way you set a different hold-music from the Queue-music by setting
both the CHANNEL(musicclass) and the queue-context musicclass.

ASTERISK-24276 #close
Reported by: Kristian Høgh
Patches:
  app_override_channel_moh.patch uploaded by Kristian Høgh (License #6639)

Review: https://reviewboard.asterisk.org/r/4010/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@423893 65c4cc65-6c06-0410-ace0-fbb531ad65f3