Add support for application/dtmf SIP INFO dtmf handling. Yep, another

way of handling DTMF in SIP. Totally undocumented, but implemented
in enough devices so we have to support it. 

Code by sergee, small changes by oej.

Closes issue #11049


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@89278 65c4cc65-6c06-0410-ace0-fbb531ad65f3