Add a function, CHANNELS(), which retrieves a list of all active channels.

(closes issue #11330)
 Reported by: rain
 Patches: 
       func_channel-channel_list_function.diff uploaded by rain (license 327)
       (with some additional changes by me, mostly to meet coding guidelines)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@120230 65c4cc65-6c06-0410-ace0-fbb531ad65f3