Make MWI in chan_skinny event based modeled after chan_zap and chan_mgcp.

(closes issue #12214)
Reported by: DEA
Patches:
      chan_skinny-vm-events-v3.txt uploaded by DEA (license 3)
	  Tested by: DEA and me


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@114700 65c4cc65-6c06-0410-ace0-fbb531ad65f3