app_queue: 'agent available' hint, cleanup restart, and initial state

Fix previously untested senarios;

1). On queue initialisation set queue_avail devstate to INUSE.
    Previously was unavailable, which indicated an agent was available.

2). When removing members, if there are no other members available, set queue_avail to INUSE.
    Previously, if a member interface had become 'unavailable', they were never going to be removed, particularly when persistant queues is enabled.

3). When adding a member, check that they are available, if they are set queue_avail to NOT_INUSE.
 Previously on reloaded, members may have been 'unavailable'.

4). When pausing or unpausing a member, set appropriate queue availability. 

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

Review: https://reviewboard.asterisk.org/r/2129/
........

Merged revisions 373804 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@373805 65c4cc65-6c06-0410-ace0-fbb531ad65f3