This adds the playing_moh flag to the conference_bridge_user struct that
signifies when MOH should be playing so code doesn't have to guess whether
MOH is playing.
This change also adds the necessary checking to ensure that MOH continues
playing for a single user in a conference after the join sound is played when
configured to do so.
(closes ASTERISK-17988)
Review: https://reviewboard.asterisk.org/r/1263/
git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@323107 65c4cc65-6c06-0410-ace0-fbb531ad65f3