Add manager command for showing all current channels.

Thanks, eliel, for writing the original patch. Modified by me to follow
other manager events and the new "moremanager" style.

(closes issue #11478)
Reported by: eliel
Patches: 
      manager.c.patch uploaded by eliel (license 64)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@91347 65c4cc65-6c06-0410-ace0-fbb531ad65f3