Add ability to hangup all channels from the CLI.

Added the keyword 'all' to the 'channel hangup request' CLI command
so that you can request all channels to be hungup without having to
restart Asterisk.

(closes issue #16009)
Reported by: moy
Patches:
      hangup-all-rev-221688.patch uploaded by moy (license 222)
Tested by: moy, russell

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@264117 65c4cc65-6c06-0410-ace0-fbb531ad65f3