chan_skinny now respects callwaiting=no

(closes issue #12691)
Reported by: sbisker
Patches:
      callwaitingv1.diff uploaded by wedhorn (license 30)
Tested by: wedhorn on old skinny phones, mvanbaak on 7960 and 7905 with latest firmware


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@138851 65c4cc65-6c06-0410-ace0-fbb531ad65f3