fix disable-proto option
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.