Switch rump from the handwritten vnode pager to the standard kernel
vnode pager.

It would have been nice to keep a separate version:
* it has helped find file system bugs which the kernel pager
  treated as non-errors
* it does not contain extra payload unnecessary in userspace

However, getting the details of the pager implementation correct
with all the flags, offsets and block/page size special cases is
*EXTREMELY* difficult (chuq > god).

On the plus side, LFS write now works for file data too instead of
just metadata.  Also, maybe being able to singlestep the genfs
vnode pager in the comfort of userspace will allow more people to
understand how the behemoth functions.

(pooka)