Add a couple of new time API calls - ast_tvdiff_sec and ast_tvdiff_usec

(closes issue #11270)
Reported by: dimas
Patches:
      tvdiff_us-4.patch uploaded by dimas (license 88)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@94029 65c4cc65-6c06-0410-ace0-fbb531ad65f3