dscp set/get

[Franco Fichtner]

> On 06 Oct 2016, at 3:32 PM, Kristof Provost <kp at freebsd.org> wrote:
> 
> OpenBSD seem to just always preserve the ECN bits (so there’s no dscp keyword).
> Perhaps we should do the same.

The following will import the OpenBSD code regarding the subject.
I retained the old manual style that is in FreeBSD to make the
additions fit in better.  Completely backwards-compatible, except
for the change in behaviour regarding to ignore ECN on set/match.

The ECN change is from 2012, so it's safe to assume it works.  ;)

https://reviews.freebsd.org/D8165


Cheers,
Franco