Remove the non-functional "swap device" userland front-end to the

Remove the non-functional "swap device" userland front-end to the
multiplexed underlying swap devices (/dev/drum).  The only thing it did
was to allow root to open /dev/drum, but not do anything with it.
Various utilities used to grovel around in here, but Matt has written
a much nicer (and clean) front-end to this for libkvm, and nothing uses
the old system any more.

The VM system was calling VOP_STRATEGY() on the vp of the first underlying
swap device (not the /dev/drum one, the first real device), and using
the VOP system to indirectly (and only) call swstrategy() to choose
an underlying device and enqueue it on that device.  I have changed it
to avoid diverting through the VOP system and to call the only possible
target directly, saving a little bit of time and some complexity.

In all, nothing much changes, except some scaffolding to support the
roundabout way of calling swstrategy() is gone.

Matt gave me the ok to do this some time ago, and I apologize for taking
so long to get around to it.