Move common code for setting T38 capabilities and fix a bug with fax detection...

Move common code for setting T38 capabilities and fix a bug with fax detection in the SIP RTP read callback. It's still sort of silly... but more on that later.
(closes issue #11239)
Reported by: dimas
Patches:
      sipt38prop.patch uploaded by dimas (license 88)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@97154 65c4cc65-6c06-0410-ace0-fbb531ad65f3