Lots of new ethernet-related code (doesn't work yet)

The board is basically implemented. It's just a dummy slot ROM
with no driver or primary/secondary init, that looks enough
like the Apple EtherTalk card. The DP8390 controller chip
has its address space implemented, and some of its registers
work. Lots more work to do on that

With slog() tracing enabled, you can see A/UX try to send a
multicast ethernet frame, then give up waiting for some kind
of response from the chip, then decide that the ethernet
controller is dead and print an error to console