Add HangupRequest manager event, to specify when/where a channel gets hung up.

(closes issue #18226)
Reported by: clegall_proformatique
Patches: 
      asterisk_1.8_293157_hanguprequests.svn.patch uploaded by clegall proformatique (license 1139)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@309300 65c4cc65-6c06-0410-ace0-fbb531ad65f3