DNID does not get cleard on a new call when using immediate=yes with ISDN signaling.

DNID does not get cleard on a new call when using immediate=yes with ISDN signaling.

When you are using chan_dahdi ISDN signaling with immediate=yes and a call
comes in without a DNID then you get the DNID of a previous call.
Chan_dahdi does not touch the DNID field on a new call if it does not have
a DNID.

Made always copy the DNID from the new call.

The patches backport the relevant changes from trunk -r210387.

(closes issue #17568)
Reported by: wuwu
Patches:
      issue17568_v1.4.patch uploaded by rmudgett (license 664)
      issue17568_v1.6.2.patch uploaded by rmudgett (license 664)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.6.2@278703 65c4cc65-6c06-0410-ace0-fbb531ad65f3