The OpenBSD sysctl code has been copy-pasted three times now. Abstract
it.

Change-Id: Ia5558927f0bc2b218b5af425dab368b5485d266c
Reviewed-on: https://go-review.googlesource.com/121775
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>