pipe2 has been available on OpenBSD for some time, all currently
supported versions of OpenBSD have an implementataion of it.
Change-Id: I740da07139541084f99a4fa4efcf67a591eed9e5
Reviewed-on: https://go-review.googlesource.com/c/sys/+/216244
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>