This means {Read,Write}Msg{UDP,IP} now work on windows.

Fixes #9252

Change-Id: Ifb105f9ad18d61289b22d7358a95faabe73d2d02
Reviewed-on: https://go-review.googlesource.com/76393
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>