Windows: Deny writes/truncation to files w RO attr

If the readonly file attribute is set on a file, refuse to process
writes, truncations or overwrites.   The afsd_service will do so
and this can lead to data corruption.

At the same time, writes from the redirector to afsd_service must
not be denied because of the readonly attribute.  That check was
performed during the CreateFile.   Otherwise, a new file can be
created with the readonly attribute and then not be writable.

Change-Id: I921a11eb8c1a3e642d60c23fc905b3febc0f0761
Reviewed-on: http://gerrit.openafs.org/10985
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>