Changes to get IP further compiling.

It's now almost possible to patch the source and do no further work.
There are some ugly sed bits still as spatch can't handle certain things.
And you need r18 spatch for the typedef and scalar scripts.

We have two nixip files that compile with more to come.

I've folded plan 9 ipv6 stuff into kern/include/ip.h, since v6 should
no longer be considered optional; and, I'm #including ip.h into plan9.h
to avoid surprises later. It can come out later if we want.

Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>