Store the cause code that is returned when trying to create a channel in ChanIsAvail in the

AVAILCAUSECODE dialplan variable instead of overwriting the device state in AVAILSTATUS.

(closes issue #14426)
Reported by: macli


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@229970 65c4cc65-6c06-0410-ace0-fbb531ad65f3