app_confbridge: Allow muting of users waiting to enter a ConfBridge

Prior to this patch, users waiting to enter a ConfBridge were not considered
when muted via the CLI or via AMI. Instead, a confusing message would be
emitted stating that the channel did not exist.

This patch allows a user to be muted when waiting to enter a ConfBridge
conference. This is equivalent to start when muted, only toggled via the CLI
or AMI.

Review: https://reviewboard.asterisk.org/r/3582

#ASTERISK-23824 #close
patches:
  rb3582.patch uploaded by tm1000 (License 6524)
........

Merged revisions 415206 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@415207 f38db490-d61c-443f-a65b-d21fe96a405b