Reuse the functions added for FreeBSD.

Change-Id: Icf5bf65c82f91147659947f6ea296dd2555724a0
Reviewed-on: https://go-review.googlesource.com/128256
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>