func_channel: Read/Write after_bridge_goto option

Allows reading and setting of a channel's after_bridge_goto datastore

(closes issue ASTERISK-21875)
Reported by: Matt Jordan
Review: https://reviewboard.asterisk.org/r/2628/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@393005 65c4cc65-6c06-0410-ace0-fbb531ad65f3