jail: Add PD_KILL to remove a prison in prison_deref().

Add the PD_KILL flag that instructs prison_deref() to take steps
to actively kill a prison and its descendents, namely marking it
PRISON_STATE_DYING, 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 is also used to clean up failed jail (re-)creations in
kern_jail_set(), which didn't generally take all the proper steps.

Differential Revision: https://reviews.freebsd.org/D28473