MFC r254776:

Make ipfw nat init/unint work correctly for VIMAGE:

* Do per vnet instance cleanup (previously it was only for vnet0 on
  module unload, and led to libalias leaks and possible panics due to
  stale pointer dereferences).

* Instead of protecting ipfw hooks registering/deregistering by only
  vnet0 lock (which does not prevent pointers access from another
  vnets), introduce per vnet ipfw_nat_loaded variable. The variable is
  set after hooks are registered and unset before they are deregistered.

* Devirtualize ifaddr_event_tag as we run only one event handler for
  all vnets.

* It is supposed that ifaddr_change event handler is called in the
  interface vnet context, so add an assertion.

Reviewed by:	zec