vol: make it clearer that SetOGM is not impl on Windows

vol: make it clearer that SetOGM is not impl on Windows

Separate the SetOGM function into two implementations to
match the other OGM functions.  Implement the Windows version
as returning -1 always.

In the future, the OGM functions could be renamed to express
their purpose: saving and restoring the volume id and tag
associated with a file.   Windows currently encodes this
information using the CreationDate and the file name but
in the future this information might be stored in an alternate
data stream.

Change-Id: I7e60cceb4548b7ad3a1e045f390e7052fece4926
Reviewed-on: http://gerrit.openafs.org/3700
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>