Implement a setting for denying/allowing transfer requests. At this stage,

we only have open/closed. Well, at least you can deny transfers from unknown
callers or at least incoming calls.
(Part of the SIPtransfer branch)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@19546 65c4cc65-6c06-0410-ace0-fbb531ad65f3