Windows: cm_BPlusDirIsEmpty

Provide a function to determine if a directory is empty or not.

Change-Id: Ib1fa642b02ad67ffdba73da1b7c6091a799de0c3
Reviewed-on: http://gerrit.openafs.org/9777
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>