Windows: Unique file ID is per volume

The unique file ID returned as part of the BY_HANDLE_FILE_INFORMATION
data structure obtained via GetFileInformationByHandle() is only
guarranteed to be unique within the volume where volume uniqueness is
determined by the volume's serial number.

It therefore doesn't make sense to return the volume id as part of
FILE_INTERNAL_INFORMATION IndexNumber.  Instead return Vnode and
Unique as that is what ensures uniqueness within an existing AFS
volume.

Unfortunately, {VolId, Vnode, Unique} does not guarantee uniqueness
for when multiple cells are in use.

Change-Id: I6c6ea4a04aa79778cce811560d8106a3a5e373cb
Reviewed-on: http://gerrit.openafs.org/9108
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>