Windows: Store OS version as a global

In DriverEntry() RtlGetVersion() is used to obtain the OS version
information.  Store the result in a global structure that can be
used elsewhere to make run time decisions based upon the OS.

Change-Id: I194e7da6858d1dea755d8de82a9bee70e63ade4d
Reviewed-on: http://gerrit.openafs.org/10350
Reviewed-by: Rod Widdowson <rdw@steadingsoftware.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Windows: EOF for Synchronous Deferred Writes on XP/2003

The Windows IO Manager is not supposed to issue multiple
outstanding cached writes to a file system for a synchronous
file object.  To do so would risk out of order application
of writes that extend the end of file and in turn risk data
corruption.  It turns out that on Server 2003 SP2 and more
than likely XP and 2000 as well, if a file system returns
STATUS_PENDING because a write was deferred due to the
Windows Cache Manager failing CcCanIWrite(), the IO Manager
will happily continue issue subsequent write requests.

On OSes older than Vista disable the use of deferred writes
and sit in a spin loop waiting for the Windows cache manager
to make room.  This is much less efficient and increases the
write latency but it is safe.

Change-Id: Ic47d62749bdb4d0475661967fcbfd25834f21a72
Reviewed-on: http://gerrit.openafs.org/10351
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>