Rename now uses MoveFileEx which was previously not available to
use because it is not supported on Windows 2000.

Change-Id: I583d029c4467c9be6d1574a790c423559b441e87
Reviewed-on: https://go-review.googlesource.com/6140Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>