Conditionally allow traffic from Guest network through VPN client
Guest network does not work while device is acting as VPN client.
It is because there is no forwarding rule in firewall, so traffic from guest network cannot go trough default route (vpn).
However, simple forwarding traffic from guest net to VPN clients zone is not the right approach. It would be better to allow only forwarding to wan and conditionally allow forwarding from Guest network to VPN clients.
Original issue: turris/os/packages#805