Copy CL 17488 into golang.org/x/sys/windows.

Change-Id: I4c6bf14982dd1fc2b8aa70ec16ad7b7c18e76bae
Reviewed-on: https://go-review.googlesource.com/73353Reviewed-by: Tim Cooper <tim.cooper@layeh.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>