app_meetme: Don't interrupt MOH for waitmarked users.

Occasionally, when the last marked user leaves the conference, waitmarked
users don't get MOH if MOH is supposed to be played while a waitmarked
user is waiting for another marked user.

* Made not interrupt MOH when the user is a waitmarked user.  The
waitmarked user doesn't need to hear any leave announcements from the
conference as the user would have already heard different leave
announcements if they were enabled.  Apparently DAHDI occasionally sends
unending non-silent streams to these users or a normal user still in the
conference has continuous high background noise.  These non-silent streams
cause MOH to be suspended while the never ending "announcement" is played.

Issue caused by ASTERISK-13680.

AST-1349 #close
Reported by: Tyler Stewart

Review: https://reviewboard.asterisk.org/r/3543/
........

Merged revisions 414401 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@414402 f38db490-d61c-443f-a65b-d21fe96a405b