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.

Signed-off-by: Alexey Zaytsev <alexey.zaytsev@gmail.com>

e1000 should now work in VMware.

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.

Cleanups for e1000.c.

I have successfully booted DOS via iSCSI and PXELINUX via TFTP with this
version of the e1000 driver.

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.

autonegotiation still needs work.  receive is not working yet.