Skinny cleanup.

Removed d->registered which was mirroring d->session. Changed relevant
references to use d->session instead.

Moved setting and unsetting of l->device from session register to device 
configuration. As such, l->device will always be valid unless it is has not
been configured to a device. Revised various test where checking if a device
is registered to use l->device->session.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@368680 65c4cc65-6c06-0410-ace0-fbb531ad65f3