Add an option for ControlPlayback to be able to start at an offset from

the beginning of the file.  Also, add a channel variable that indicates
the location in the file where the Playback was stopped.
(closes issue #7655, patch from sharkey)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@68502 65c4cc65-6c06-0410-ace0-fbb531ad65f3