mtree: Add missing directories generated by certctl

This is a direct commit to the stable/13 branch, cherry-picking commit
5b7f73ce16cb ("mtree: Add missing directories generated by certctl"),
because the "untrusted" directory is called "blacklisted" on stable/13.

PR: 255639
Reviewed by: imp
Approved by: emaste (mentor)
Differential Revision: https://reviews.freebsd.org/D49037