Windows: GetVolumeInfo cell name

The service was supposed to provide the redirector the cell name
as part of the AFSVolumeInfoCB response from RDR_GetVolumeInfo.
Add a new field to store it and populate it.

Change-Id: Idcafaee66ed1031dfdeee1804bbdff617e464d3f
Reviewed-on: http://gerrit.openafs.org/9079
Reviewed-by: Rod Widdowson <rdw@steadingsoftware.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>