Windows: cm_user interlocked operations

Convert cm_user refcnts to use interlocked operations.

Change-Id: Ib40d2446792dcde3038ea601281406e013147595
Reviewed-on: http://gerrit.openafs.org/9061
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>