Convert chan_skinny's open-coded linked lists to the list macros

(closes issue #12956)
Reported by: DEA
Patches:
      chan_skinny-linkedlists-v2.txt uploaded by DEA (license 3)
Tested by: DEA, mvanbaak



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@130576 65c4cc65-6c06-0410-ace0-fbb531ad65f3