Add support for a=inactive in SDP for holding. This is not very well documented

in RFCs, as they keep referring to each other in a circular pattern in regards
to this item, but since the Nokia SIP/GSM phones use this, we might as well
start supporting it.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@36252 65c4cc65-6c06-0410-ace0-fbb531ad65f3