Adding this helper enables me to eliminate some nasty code which
invokes unix.Syscall6 directly, and which has 32-bit shims for
invoking setsockopt properly on 32-bit Linux.

Change-Id: I53ef411e4c96cef557999261760cddc00df58d8a
Reviewed-on: https://go-review.googlesource.com/c/163303
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>