swap_pager.c and related commits.
Essentially swap_pager.c is backed out to before the changes, but
swapdev_vp is converted into a real vnode with just VOP_STRATEGY().
It no longer abuses specfs vnops and no longer needs a dev_t and
/dev/drum (or /dev/swapdev) for the intermediate layer.
This essentially restores the vnode interface as the interface to the
bottom of the swap pager, and vm_swap.c provides a clean vnode interface.
This will need to be revisited when we swap to files (vnodes) - which
is the other reason for keeping the vnode interface between the swap pager
and the swap devices.
OK'ed by: dillon