--no-legend is replaced by --legend=no.
--quiet now implies --legend=no, but --legend=yes may be used to override that.
--quiet controls hints and warnings and such, and --legend controls just the
legends. I think it makes sense to allow both to controlled independently, in
particular --quiet --legend makes sense when using systemctl in a script to
provide some user-visible output.
Fixes#18560.
Does what the name suggests. Obviously inspired by sudoers, but note that
our tools are not supposed to be installed suid, so there is no privilege
boundary to cross here.