confbridge: Make kick, mute and unmute handle channel targets consistently.

Kick, mute and unmute were a little inconsistent in their handling of channel
targets.  This patch cleans that up by insuring they all handle the 'all'
target consistently and adds the 'participants' target which acts on
non-admins.  Documentation for kick was also cleaned up as it never
supported partial channel names.

Tested by: George Joseph
Review: https://reviewboard.asterisk.org/r/3944/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@422090 f38db490-d61c-443f-a65b-d21fe96a405b