Add "Who Hung Up?" implementation for DAHDI SS7 subtechnology

Testing was done on a local machine to verify that protocol and
cause information was being sent properly.

Review: https://reviewboard.asterisk.org/r/1955/
(issue SWP-4222)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@369184 65c4cc65-6c06-0410-ace0-fbb531ad65f3