Improve our triggering of a T38 switchover internally when triggered by a received...

Improve our triggering of a T38 switchover internally when triggered by a received reinvite.

Previously we reached across the channel bridge to get the other party's SIP dialog
structure in order to trigger an outgoing reinvite. This is extremely dangerous to do
and only works if bridged to another SIP channel. This patch changes this to use the
T38 control frame method of requesting a switchover. This change also causes the SIP
channel driver to propogate back whether the switchover worked or not instead of blindly
accepting the incoming T38 reinvite.

Review: http://reviewboard.digium.com/r/200/

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@183108 65c4cc65-6c06-0410-ace0-fbb531ad65f3