Windows: Add validation for directory buffer contents

Windows: Add validation for directory buffer contents

If the directory buffer contents are garbage we can crash
the service.  Add some simple validation checks to ensure
that cm_dirEntry_t objects have the correct flag value and
that the name strings are not too long.

LICENSE BSD

Change-Id: If4a276007ff7a21a641825037a1035ea20db79c5
Reviewed-on: http://gerrit.openafs.org/2658
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>