[9fans] coraid ethernet console

[erik quanstrom <quanstro@coraid.com>]

coraid ethernet console (cec) is a way to do most of what you
can do over a serial console over raw ethernet frames.

coraid appliances use cec instead of a serial console so we can
have console access without tcp, ip or a serial connection.
i've put our plan 9 client in /n/sources/contrib/quanstro/cec.tar.
there's a protocol description and a man page in that tar.

i've also submitted a port of the cec device to the cpu kernel.
/n/sources/patch/cpu-cec.  unfortunately as i'm writing this, 
(i realize that i forgot to write a manual page.  i'll fix that in the
next few days.)

i've used this driver to debug a couple of device drivers.
we don't always have enough ports on the console server to go
around.  generally, cec will give you all the output that would be 
available on the serial console once plan 9 has started.  it
definately beats sitting in the unix room.

limitations (or future work)

1.  consolefs doesn't yet speak cec.  (good soc project.)
2.  the cec client doesn't know how to listen for the Toffer
sent by the kernel driver when it starts listening.
3.  thus, 9load doesn't speak cec.
4.  i didn't submit my boot time modifications that make
cec available on boot.

due to a limitation in netif, only one cec client may
run per interface.

enjoy
- erik