Windows: replace CYGWIN envvar with CYGWINDIR

Windows: replace CYGWIN envvar with CYGWINDIR

The environment variable CYGWIN (starting with cygwin 1.7.1) is
now used by CYGWIN to set configuration parameters for the cygwin
runtime library.  OpenAFS used it to indicate the location of the
Cygwin install directory.  Since there is a conflict, rename CYGWIN
to CYGWINDIR.

Change-Id: I98f6a7095ede4c6a91dd71d48312b0d1a55b8091
Reviewed-on: http://gerrit.openafs.org/4629
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>