This change just picks a few constants from DragonfFly BSD 4.6 kernel
and doesn't synchronize all the existing constants with the latest
DragonFly BSD kernels.

Updates #14222.

Change-Id: Ie107a8bee1a09393b3b42b6f82489532f5d13290
Reviewed-on: https://go-review.googlesource.com/40894Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>