Windows: Convert BkgDaemon Procedures to use rock

The BkgDaemon functions accepted four 32-bit parameters into
which 64-bit offsets and lengths were fit.  Convert the routines
to use a procedure specific rock type which is allocated by the
caller to cm_QueueBkgRequest and freed by cm_BkgDaemon.

The use of a rock will permit greater flexibility for future
background procedures to be implemented that require more complex
data to be passed around.

Change-Id: I4fc06e1d4a3a3ee25c699c95a90a9c9993199ddd
Reviewed-on: http://gerrit.openafs.org/8966
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>