NetBSD uses sysctl with struct clockinfo to get clock rate information
from the kernel. Add type Clockinfo and the SysctlClockinfo function
to query this information.

This will be used in github.com/tklauser/go-sysconf to get _SC_CLK_TCK
on NetBSD.

Change-Id: I9e67d766f491ec3b460f26cb243b3595f0ba4d69
Reviewed-on: https://go-review.googlesource.com/138035
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>