packages: Add minimal-jail set

This is minimal, but without bootloader, hardware and networking support
that's typically not required in jails.

This requires extending the 'set' annotation to be a comma-separated
list, so that packages can be in multiple sets.

MFC after: 3 seconds
Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D52591

(cherry picked from commit 9b7bddfd082d1df2b4c97e7ec974d1d67e98a1de)