Stop dying jails from become alive again via races in fork(2) and jail_attach(2), and by the implicit rebirth of parent jails when a child jail is added when they're dying. Explicit rebirth the documented way, via the JAIL_DYING flag in jail_set(2), is still allowed.
The jail state and user reference count are a little more out of sync, with jail_remove(2) setting the state to dying before all the user references (processes) are gone. This makes pr_state the clear indicator of jail aliveness.