Remove all OpenBSD/NetBSD code. It was the right place to start from, but
it now really gets in the way.

This allows us to fix several problems- not least of which was problems
of ordering about when you'd have a device softc for an miibus child
available or not. Move some steps of things around.

Put the ifnet/arpcom structure at the head of the softc (PR 29249).

Don't do tx gc in the interrupt service routine- that seems to make
things a bit more efficient.

Enable jumbo support by default- but this version of 'jumbo' is broken
because it really is just using multiple tfd/rfd's to match a packet,
which will never be > CLSIZE anyway.

This should begin the first steps toward cleaning this driver up.

PR: 29249
MFC after: 1 week