fix for nullfs users

[Ted Unangst <tedu@Stanford.EDU>]

Here's a patch that should correct most of the issues with nullfs,
unionfs, and so on.  It basically takes the new NetBSD nullfs, patches vfs
to work with it, and then patches the other filesystems to work with the
new vfs.  Not everything is tested, and nfs and xfs aren't touched, so
count them as broken.  That will come a little later. But now you can run
apache chroot. :)

It seems pretty stable.  At least, it's an improvement.  And the fact that
the original author of this code thought through what locking was needed
and what might happen is encouraging.  I ran some tests doing funny stuff
with recursive null mounts, but no problems.  If you can crash it, please
tell me how.

The initial release only has two bugs. :)
1.  null mounting over a union mount will panic because of recursive
locking.  doesn't impact ability to use either alone though.
2.  null mounting a file works, but you can't unmount it.
Short answers: don't do that.  I have an idea what's going on, but I won't
fix it for a while.  (famous last words.)

Some parts of this patch, particularly in the lesser used fs's like
portal, are pulled from Peter Werner's earlier patch.  Thanks.
http://archives.neohapsis.com/archives/openbsd/2002-06/0218.html

This should apply against today's current.
http://www.stanford.edu/~tedu/vfs.diff -- 280k be warned.
or http://www.stanford.edu/~tedu/vfs.html


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