Add option to wait to be able to explicitly send ACM via the Proceeding()...

Add option to wait to be able to explicitly send ACM via the Proceeding() application in the dialplan.  Also minor documentation update explaining how to setup multiple signalling links within a linkset

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@128122 65c4cc65-6c06-0410-ace0-fbb531ad65f3