prison_check(9): Bring up-to-date with hierarchical jails

Reviewed by: bcr, emaste, pauamma_gundo.com, mhorne
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D40639

(cherry picked from commit e9fdd494537ca45b14e0917e8bb1595b6460f3a3)

Approved by: markj (mentor)