allow setting a channel variable to disable sending SIGHUP to the AGI process

(issue #6491, original patch by juggie, channel variable patch by corydon,
 committed patch modified to change variable name and update documentation)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@30337 65c4cc65-6c06-0410-ace0-fbb531ad65f3