Add ability for network devices to flag link up/down state to the
networking core.
Autobooting code will now wait for link-up before attempting DHCP.
IPoIB reflects the Infiniband link state as the network device link state
(which is not strictly correct; we also need a succesful IPoIB IPv4
broadcast group join), but is probably more informative.
The e1000_irq() routine should (per mcb30) do enable on non-zero,
disable on zero. This is not consistent in all drivers, so I'll
wait to update it when doing a global sweep.
There may still be an issue with memory handling, since it seems to
die ungracefully when ARP packets come in after loading a kernel.
Something to debug.
