devfs - Let devfs assume degenerate knotes when a device goes away

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