Add a new feature for Music on Hold. If you set the "digit" option for a

class in musiconhold.conf, a caller on hold may press this digit to switch
to listening to that music class.
  This involved adding a new callback for generators, which allow generators
to get notified of DTMF from the channel they are running on.  Then, a callback
was implemented for the music on hold generators.
(patch from bbryant)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@65505 65c4cc65-6c06-0410-ace0-fbb531ad65f3