- Don't let rman_reserve_resource() activate the resource in
  nexus_alloc_resource() and don't set a bushandle.
  nexus_activate_resource() will set a proper bushandle.
- Implement a proper nexus_release_resource().
- Fix ixppcib_activate_resource() to call rman_activate_resource()
  before creating a mapping for the resource.

Tested by:	jmg