unionfs port/update

[Joe Talbott]

I've been digging around a bit in the nullfs and unionfs code.  So far
I have determined that I will need to implement nresolve and friends.

What do you think is the best approach:

1. Start from scratch using nullfs as a guide.

or

2. Build from what we have (which is old) using nullfs as a guide.

I haven't yet read the unionfs papers so the theory is still very
generalized in my head.  Basically as I understand it all writes
should go to an upper level vnode and reads should try the upper level
first and fall through to the lower level if there is no upper shadow
copy.  Removes should always operate on the upper level as well as
creates and renames.

Please let me know your thoughts and most importantly any tips and
advice you may have.

Thanks,
Joe