Add SLA to skinny.
Adds sublines to skinny lines. Each subline can be attached to an
SLA station/trunk combo. Includes the following functionality:
Callid is persistent for both in/out calls on all skinny devices.
Can join, hold, resume.
All sublines appear under a single line button.
See: https://wiki.asterisk.org/wiki/display/~wedhorn/Skinny+SLA for doc.
(closes issue ASTERISK-17947)
Review: https://reviewboard.asterisk.org/r/1239/
git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@328381
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