Add the PD_KILL flag that instruction prison_deref() to take steps to actively kill a prison, namely clearing its PR_PERSIST flag and killing any attached processes.

This replaces a similar loop in sys_jail_remove(), bringing the operation under the same single hold on allprison_lock that it already has. It also replaces a bit of one-off code in kern_jail_set() to handle the destruction of almost-finished prisons.