Windows: unified afs errors must use nt mapping

Windows: unified afs errors must use nt mapping

On Windows, error.h does not provide a complete list of POSIX
C99 error values.  OpenAFS fills in the gaps with a private
error mapping table afs/errmap_nt.h (src/util/errmap_nt.h).
If errmap_nt.h is not included prior to processing unified_afs.h,
values such as ELOOP will be mapped to EIO instead of the unique
value defined by errmap_nt.h.

Change-Id: I3d25fafb15f1b0060ad1047178cbadd5ce22edb2
Reviewed-on: http://gerrit.openafs.org/5128
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>