Move the DAHDI-to-DAHDI operator mode check from app_dial into chan_dahdi

so we don't have to hardcode anything.

(closes issue #13636)
Reported by: seanbright
Patches:
      13636.diff uploaded by seanbright (license 71)
Reviewed by: russellb, putnopvut


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@147388 65c4cc65-6c06-0410-ace0-fbb531ad65f3