The default value for disable-proto is {'udp', 'udp6'}, and it's not possible to get { } as a final value for disable-proto when passing option disable-proto <proto>, because this results in disable_proto being equal to {'udp', 'udp6', }. Similarly, passing both disable-proto <proto> and disable-proto none results in the none overwriting anything else, resulting in disable_proto being an empty list. The goal here is to allow to only keep the protocol we want to disable when passing disable-proto <proto>, the default 'udp' and 'udp6' are in this case only added to disable_proto if no disable-proto option is found.

/cc @tomo @jm