Windows: always try afs/cell@USER-REALM first

Windows: always try afs/cell@USER-REALM first

In the KFW_AFS library, always try afs/cell@USER-REALM
first, even when KFW_AFS_klog() is called with an explicit
realm mapping for the cell.  An afs service principal from
the user's realm is always preferred.  No cross realm and
if the realm is AD, the ability to avoid the inclusion of
a PAC.

Change-Id: Ia29085e03d7c8a7c05e0c8d7991bc48b780b84fa
Reviewed-on: http://gerrit.openafs.org/4633
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>