[9fans] bugs, races, more bugs

[David Presotto <presotto@closedmind.org>]

Timing out arp entries uncovered some bugs that we
hadn't noticed before plus I added one with the
timeouts.  The symptom was quiescent connections
timing out.  Fixed in ip/arp.c.

We also found a few races between packets being rerouted
and IP stacks being torn down.  The symptom was
a null dereference in ip.c crashing the kernel.
Fixed in ip/ip.c, ip/ipifc.c, ip/iproute.c,
ip/ipv6.c.  I left an assert in ip.c to make
the race more obvious in case we haven't stomped
on it yet.

Finally, I updated port/proc.c and port/devproc.c
with suggestions on scheduler changes from the list.
Processes trapped at low priority levels now float
up.  Also, sleep(0) now drops a process' priority
one level each call so that processes doing something
like:

	while(condition not true)
		sleep(0);

should dip down below whatever its waiting for.
I've been testing it on all our systems for a few
days and haven't seen a problem.  However, I never trust
this stuff till a wider audience tries it.  Could
people that didn't do well with the last version
try this out soon?