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: I32cd79eba33fc80e15308010fafdaecb527d23f0
Reviewed-on: http://gerrit.openafs.org/4629
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-on: http://gerrit.openafs.org/4630
Tested-by: Jeffrey Altman <jaltman@openafs.org>