Properly handle UAC/UAS roles for SIP session timers

Properly handle UAC/UAS roles for SIP session timers

The SIP session timer mechanism contains a mandatory 'refresher' parameter
(included in the Session-Expires header) which is used in the session timer
offer/answer signaling within a SIP Invite dialog. It looks like asterisk is
interpreting the uac resp. uas role only as the initial role of client and
server (caller is uac, callee is uas). The standard rfc 4028 however assigns
the client role to the ((RE)-Invite) requester, the server role to the
((RE)-Invite) responder.

This patch has Asterisk track the actual refresher as "us" or "them" as opposed
to relying on just the configured "uas" or "uac" properties.

(closes issue AST-922)
Reported by: Thomas Airmont

Review: https://reviewboard.asterisk.org/r/2118/
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Merged revisions 373652 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 373665 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 373690 from http://svn.asterisk.org/svn/asterisk/branches/11

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@373691 65c4cc65-6c06-0410-ace0-fbb531ad65f3