bug#63072: 28.2; CC Mode: Fix "bsd" style and add "freebsd" and "openbsd" ones

[Eli Zaretskii <eliz@gnu.org>]

> From: Olivier Certner <ocert.dev@free.fr>
> Cc: 63072@debbugs.gnu.org
> Date: Wed, 26 Apr 2023 09:28:17 +0200
> 
> > If documented that these styles change to follow the corresponding
> > projects, it might be okay, but do we really want to commit ourselves
> > to follow them from now to eternity?  That's a non-trivial maintenance
> > burden, unless the projects themselves take that up upon themselves,
> > and will be submitting changes whenever their conventions change.
> 
> But I never said we would commit to that. On the contrary, what I would like 
> is that the documentation says clearly that "freebsd" and "openbsd" may 
> change, so that people are aware they cannot rely on these styles to always 
> remain the same (contrary to the old styles). So that's rather a non-
> commitment.
> 
> Additionally, these changes will be exceedingly rare, and styles will just be 
> updated on a best-effort basis.

The "best-effort" part is what bothers me.  We introduce these new
styles because the relevant projects change the styles, and then we
basically tell users: don't expect these styles to actually follow
those projects, except by luck?  Does that make sense?