func_channel: Add CHANNEL(onhold) item to get the current hold status of the channel.

It would be useful to get the current hold status of a channel.

Added CHANNEL(onhold) item that returns 1 (onhold) and 0 (not-onhold) for
the hold status of a channel.

ASTERISK-24038
Reported by: Matt Jordan

AFS-113 #close
Reported by: Mark Michelson

Review: https://reviewboard.asterisk.org/r/3983/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@422870 65c4cc65-6c06-0410-ace0-fbb531ad65f3