This change makes use of new routing message APIs for BSD variants to
support FreeBSD 11 and newer versions of other BSDs.

Fixes #7849.
Fixes #14724.

Change-Id: I56c7886d6622cdeddd7cc29c8a8062dcc06216d5
Reviewed-on: https://go-review.googlesource.com/22451Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>