Also makes it possible to capture RTM_DESYNC messages when you have a
routing protocol implementation that requires to be aware of the
health condition of kernel RIB on OpenBSD.

Change-Id: Idd0c8c5e8f5ea72a4d56c9a46c137786bcda6354
Reviewed-on: https://go-review.googlesource.com/50191
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>