needed to access the internals of buffers but not necessarily to use
the VOP. <sys/buf.h> recently grew a bogus dependency on <sys/systm.h>
for the declaration of spl*, and I prefer to fix the synopsis breakage
by removing a wart instead of adding one.
incomplete and some are just placeholders but I wanted to try to get
something at least into 2.2 on the grounds that what I have is a lot
better than nothing. I also wanted to commit something which documents
the interfaces in 2.2 before I start updating the documentation for 3.0.
This is a definite 2.2 candidate and is also relavent to 2.1 if people
still care about that branch.
