This replaces the previous placeholder routing layer with a real one!
It's still very primitive, doesn't deal with things like timeouts very
well, and will probably need several more iterations to support more
normal networking things.
I haven't confirmed that this works with anything other than the QEMU
user networking layer, but I suspect that's what nearly everybody is
using at this point, so that's the important target to keep working.
Choosing adapter for transmit is done by adapter_for_route_to(IPv4Address).
This is just hard-coded logic right now but can be expanded to support a
proper routing table.
Also start moving kernel networking code into Kernel/Net/.
