machine: Pass machine, user, program values to polkit on OpenMachineShell

This allows more granular access control in PolicyKit rules, similar to
/etc/sudoers, for polkit actions:
* org.freedesktop.machine1.host-shell
* org.freedesktop.machine1.shell

Example configuration, place in /etc/polkit-1/rules.d/

polkit.addRule(function(action, subject) {
    if (action.id == "org.freedesktop.machine1.host-shell"
        && subject.user == "my-user"
        && action.lookup("user") == "target-user") {
            return polkit.Result.YES;
    }
});