jail: re-commit 811e27fa3c44 with fixes

Make sure PD_KILL isn't passed to do_jail_attach, where it might end
up trying to kill the caller's prison (even prison0).

Fix the child jail loop in prison_deref_kill, which was doing the
post-order part during the pre-order part. That's not a system-
killer, but make jails not always die correctly.