subr_disk - 'official' support for cloning; cleanup

subr_disk - 'official' support for cloning; cleanup

* Add 'official' cloning support, so that the top level node is also
  make_only_dev'd in the case of a cloned disk creation. This way
  the top level node can be used as a return value for the clone
  handler.

* Right now we are basically returning the raw underlying device node
  when we clone in vn. This can now be changed to actually do things
  properly so that everything passes through the disk subsystem.

* Clean up some dead code mostly related to the old understanding of
  'cloning', which has nothing to do with what's going on nowadays.

* Fix a leak of cdevs. The underlying 'rawdev' was never being released,
  resulting in a leak of these each time a disk was destroyed.