Debian-based configuration for wireguard

[Jason A. Donenfeld]

>
> fwiw, some of us do also run debian systems without ifupdown these days.
> I'm looking forward to systemd-networkd integration personally :)
>
>         --dkg

I really need to leap into action and start on the Netlink stuff,
which is the only thing currently blocking the systemd-networkd
integration. It's been hard to get motivated for this kind of thing,
but it seems like everything everyone wants to do with WireGuard is
waiting for it, so I should probably get to it soon...