devfs - Let devfs assume degenerate knotes when a device goes away
* Some devices can disappear while knotes are still active on the klist
embedded in the devices softc or etc., create a mechanism to allow devfs
to assume these degenerate knotes.
* Convert ums to this new way of things
* This does not actually fix the reported panic (where X is holding a mouse
open across detach), panic is moved into the new devfs handler.
* Other devices which could be detached and support the kq interface need to
use this interface as well (umass, ...)
Reported-by: Rumko