core: pass details to polkit for some unit actions

core: pass details to polkit for some unit actions

The following details are passed:

- unit: the primary name of the unit upon which the action was
        invoked (i.e. after resolving any aliases);
- verb: one of 'start', 'stop', 'reload', 'restart', 'try-restart',
        'reload-or-restart', 'reload-or-try-restart', 'kill',
        'reset-failed', or 'set-property', corresponding to the
        systemctl verb used to invoke the action.

Typical use of these details in a polkit policy rule might be:

  // Allow alice to manage example.service;
  // fall back to implicit authorization otherwise.
  polkit.addRule(function(action, subject) {
      if (action.id == "org.freedesktop.systemd1.manage-units" &&
          action.lookup("unit") == "example.service" &&
          subject.user == "alice") {
          return polkit.Result.YES;
      }
  });

We also supply a custom polkit message that includes the unit's name and
the requested operation.