packages: Put dhclient in the minimal-jail set

This is somewhat widely used in VNET jails, it's fairly small (150kB on
amd64) and it's enough of a core system component that it's reasonable
to include, even if many jails don't require it.

MFC after: 3 days
Reviewed by: dch
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D53154

(cherry picked from commit 8a9d29bedab597df6bfd4fab1b1bb023f8375a1f)