confbridge: Separate user muting from system muting overrides.

The system overrides the user muting requests when MOH is playing or a
waitmarked user is waiting for a marked user to join.  System muting
overrides interfere with what the user may wish the muting to be when the
system override ends.

* User muting requests are now independent of the system muting overrides.
The effective muting is now the logical or of the user request and system
override.

* Added a Muted column to the CLI "confbridge list <conference>" command.

* Added a Muted header to the AMI ConfbridgeList action ConfbridgeList
event.

(closes issue AST-1102)
Reported by: John Bigelow

Review: https://reviewboard.asterisk.org/r/2960/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@402425 f38db490-d61c-443f-a65b-d21fe96a405b