Adds the bind-to-interface helper code

[farcaller]

Typical UDP socket would listen on 0.0.0.0:67 and bind to an interface
to limit exposure. This PR implements a helper method to bind and serve
on a socket.

Currently it’s linux only. I left an unimplemented!() trap for BSD with
a note how to progress there and feature-gated the code.

[krolaw]

In my Go version, I went with the equivalent of bind_and_serve (called ServeIf), which I now believe was a mistake. In my private branch I've deprecated those functions - favouring a NewUDP4BoundConn(ifname string, addr string) fn, the result of which is passed to Serve. However, now I'm thinking, that such a general function deserves it's own lib, leaving the DHCP4r completely OS agnostic.

[farcaller]

While I can agree that it could fit net2 or similar better, I'd argue that it's widely used for dhcp servers specifically, so all of the users would end up re-implementing it over and over. Given it's feature and platform gated, I think it's still an useful addition.

[farcaller]

bump.