Error on attempts at writing martian packets

[reynir]

If we try to write to a loopback address we error out. This is perhaps not ideal, but sending IP packets with loopback addresses is not allowed according to RFC 1122.

I'm not sure what to do with such packets. Ideally, they would instead be written to a loopback interface, but I don't think we have that.

Something similar should likely be done for ipv6, and there may be other addresses which we need to handle differently.

See also #537

[hannesm]

Thanks, looks good to me.

[reynir]

The latest commit basically adds if dst = ::1 then Error something else in the beginning of write in the IPv6 implementation. I'm not sure that's sufficient as there might be other places where routing is done. Changing the neighbor discovery code to implement this seemed less straightforward. So maybe we should drop that commit and think about other implementations?