- add support for setting an AGISTATUS variable that indicates successful

  execution, failure, or if the channel requested hangup.
- only return -1 from the application if the application requested hangup. If
  there was just a failure in execution of the AGI, just set the status
  variable appropriately and move on in the dialplan.
(issue #7121, original patch by Alessandro Polverini, updated patch by srt,
 committed patch is heavily modified to allow still returning -1 on hangup)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@30272 65c4cc65-6c06-0410-ace0-fbb531ad65f3