Make the Hangup application set a softhangup flag.

The Hangup application used to just return -1 to cause normal dialplan
execution to hangup a channel.  For the non-normal execution routines like
predial and connected-line interception routines, the hangup request would
exit the routine early but otherwise be ignored.

* Made the Hangup application not allow setting a cause code of zero.  A
zero cause code is not defined.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@368979 65c4cc65-6c06-0410-ace0-fbb531ad65f3