Support hold/unhold in Zap, update IAX2 parser to know about modern commands,...

Support hold/unhold in Zap, update IAX2 parser to know about modern commands, forward hold/unhold in dial, add hold device state 
and implement holding in the SLA.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@37318 65c4cc65-6c06-0410-ace0-fbb531ad65f3