Parse global variables or expressions in hint extensions

Parse global variables or expressions in hint extensions. Like: exten => 400,hint,DAHDI/i2/${GLOBAL(var)}

(closes issue #16166)
Reported by: rmudgett
Tested by: mnick, rmudgett


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@233093 65c4cc65-6c06-0410-ace0-fbb531ad65f3