Windows: record mount point string data version

Windows: record mount point string data version

The Windows cache manager stores the mount point or symlink target
string in the cm_scache_t object.  If the string is the empty string
then the target needs to be resolved.  Otherwise it is considered
up to date.  With this approach, care must be taken to ensure that
the string is erased whenever the data version changes.

This patchset records the data version of the mount point target
string in the cm_scache_t object.  Being up to date is determined
by comparing the current data version of the object to the mount
point string version.  A match and the string is up to date.

Change-Id: I4dfdc1af5894548afb35e84e77f7f607674bd7af
Reviewed-on: http://gerrit.openafs.org/7745
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>