FreeBSD if_wg POINTTOPOINT and MULTICAST behaviour

[Jason A. Donenfeld]

Hey Stefan, Toke,

I spent the day playing around with bird and babel and sorted out
FreeBSD's v6 situation. Basically, ff00::/8 addresses are treated
differently, and they're blocked unless the interface sets
IFF_MULTICAST. So I've committed
https://git.zx2c4.com/wireguard-freebsd/commit/?id=a7a84a17faf784857f076e37aa4818f6b6c12a95
to do this. We _could_ also set IFF_BROADCAST, which would translate
to babel enabling IF_MULTICAST, but so far I can't see how this would
help anything real, and combined with Toke's patch --
https://bird.network.cz/pipermail/bird-users/2021-April/015415.html --
I think we're actually in a good situation. Seeing that this now
works, I've also dropped the link1 hack and put that in a branch in
case it becomes useful later.

Bernhard (decke@) is CC'd here in case he'd like to get some of this
into ports early for your use case. Specifically, this involves:

1) https://bird.network.cz/pipermail/bird-users/2021-April/015415.html
for the bird2 package.
2) https://git.zx2c4.com/wireguard-freebsd/patch/?id=a7a84a17faf784857f076e37aa4818f6b6c12a95
for the wireguard-kmod package.

Stefan - please let me know if those work for you. In my testing thus
far, things seem to work for me.

Long term, we'll certainly want to have Toke's planned support for
direct WireGuard peering inside of bird.

Regards,
Jason