If 'faxdetect=yes' in sip.conf, switch to a 'fax' extension (if it exists)...

If 'faxdetect=yes' in sip.conf, switch to a 'fax' extension (if it exists) after T38 is negotiated.  
Terry Wilson created the original patch for this functionality, which I slightly modified and added 
the faxdetect=yes|no configuration option.  This patch is only for T38 fax detection and does not 
do anything for G711 over SIP fax detection.  By default, this option is disabled. 

Reviewboard: http://reviewboard.digium.com/r/69/

This functionality is for issue AST-140.




git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@161115 65c4cc65-6c06-0410-ace0-fbb531ad65f3