confbridge: Separate user muting from system muting overrides.

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/

Cherry-picked to support ASTERISK-20987

Change-Id: Iec7bd77736847af6a5f70b8d279f85c6e2082ee2