Starting with 1.12, we must use syscall versions
of sysctl instead of the raw syscall.

An identical CL will go into the vendored copy of this package in the stdlib.

Change-Id: I6286ab3e49f82512491afb5bcf349e89ab5645ab
Reviewed-on: https://go-review.googlesource.com/c/148597
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>