nnull

[Csaba Henk]

Hi!

I just posted two patches to submit@ which make nullfs work with the new
namecache API (yes, it's without vnode overlaying).

One is just the nullfs stuff, the other has a somewhat broader effect:
a rewrite of umount() which uses namecache rather than vnodes. It's
necessary for getting this style of lite overlay work, and as far as I'm
concerned, it's fine for other fs-es.

I didn't care about NFS export stuff.

I also noticed that umapfs is compiled regardless of the (non-)presence
of 'option UMAPFS' in the config file. (I didn't trace down the reason
for this.) With the old nullfs code, this just yielded an unloadable
module, with the new one, it won't even compile. So please fix the
Makefiles :)

Regards,
Csaba