vpn: try to launch polkit auth agent if no one found.
In the legacy bitmask client, we had a function that, when no polkit agent is found, tried to launch whatever polkit auth agent is found in the system. We could just port that code.
In the legacy bitmask client, we had a function that, when no polkit agent is found, tried to launch whatever polkit auth agent is found in the system. We could just port that code.