Change printf() calls to log() calls.

vinumattach: Zero out tables after allocating them

Modify procedure at unload: if a vinum(8) has the superdev open, don't
close down. If only the daemon has it open, send the daemon a stop
request and wait for it to close the superdev, then unload.

In order to do this, create a second superdev which is opened by the
daemon. The open and close routines set a different bit in
vinum_conf.flags; otherwise the treatment is identical.

Remove opencount field in vol structure; replace by a flag bit, since
we can't count the number of opens.

Remove dead LKM grunge.