Check dual-stack support for OpenBSD and other platforms

[stv0g]

// BUG(rsc,mikio): On DragonFly BSD and OpenBSD, listening on the
// "tcp" and "udp" networks does not listen for both IPv4 and IPv6
// connections. This is due to the fact that IPv4 traffic will not be
// routed to an IPv6 socket - two separate sockets are required if
// both address families are to be supported.
// See inet6(4) for details.

Source: https://go.dev/src/net/ipsock.go

Workaround: Open two sockets for DragonFly and OpenBSD.

This issue does not seem to affect Windows.

[stv0g]

We now support listening on multiple addresses to for all platforms.

If no listen address is configured explicitly, we automatically listen on all available addresses.

In this case:

• OpenBSD and DragonFly BSD will open two sockets. One for IPv4 (udp4) and another one for IPv6 (udp6)
• Other platforms open a UDP socket which receives traffic from both address families (udp)

We don't support listening on multiple addresses of the same address familiy yet. In order to support this, we must keep track over which socket we have received data from a peer to choose the correct socket.

Open question: which socket should be used if we initiate a handshake and have not received data from a peer?

Related: https://github.com/rosenpass/rosenpass/issues/77