CL 125456 added the implementation of AF_UNIX to the syscall package.
Add the same implementation to golang.org/x/sys/windows as well.

This works only on Windows 10.

https://blogs.msdn.microsoft.com/commandline/2017/12/19/af_unix-comes-to-windows/

Updates golang/go#26072

Change-Id: Ibd4a17342ed4f9f4f2b16b82c7b02834e681b7b4
Reviewed-on: https://go-review.googlesource.com/132555
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Yasuhiro MATSUMOTO <mattn.jp@gmail.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>