loginctl: convert to sd-bus

NOTE: the show-* subcommands do not print some properties:
  this are those with types like (so), a(so), (uo),...
  we need to fix this, but I'm not sure how
systemctl: we show headers for list-units even when piped these days, hence do the same for list-jobs
polkit: don't spawn local client if we access a remote system