app_queue: Support an 'agent available' hint

app_queue: Support an 'agent available' hint

Sets INUSE when no free agents, NOT_INUSE when an agent is free.

modifes handle_statechange() scan members loop to scan for a free agent
and updates the Queue:queuename_avial devstate.

Previously exited early if the member was found in the queue.

Now Exits later when both a member was found, and a free agent was found.

alecdavis (license 585)
Reported by: Alec Davis
Tested by: alecdavis

Review: https://reviewboard.asterisk.org/r/2121/

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@373188 65c4cc65-6c06-0410-ace0-fbb531ad65f3