res_agi: Add AGIEXITONHANGUP variable.

This patch adds a variable AGIEXITONHANGUP for res_agi.  If this variable is
set to "yes" on a channel, AGI() will exit immediately once a channel hangup
has been detected.  This was the behavior of AGI() in Asterisk 1.4 and earlier
and is still desired by some people.

Review: https://reviewboard.asterisk.org/r/1734/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@355102 65c4cc65-6c06-0410-ace0-fbb531ad65f3