Add option to get untruncated channel name from AGENT function.

The "channel" option would chop the channel name at the last '-', which made
it useless for something like a channel transfer from the dialplan.  The
"fullchannel" option will return the channel name as-is.

ABE-2218


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@270260 65c4cc65-6c06-0410-ace0-fbb531ad65f3