MINOR: config: make sure never to mix dgram and stream protocols on a bind line

MINOR: config: make sure never to mix dgram and stream protocols on a bind line

It is absolutely not possible to use the same "bind" line to listen to
both quic and tcp for example, because no single transport layer would
fit both modes and we'll need the type to choose one then to choose a
mux. Let's make sure this does not happen. This may be relaxed in the
future if we manage to instantiate transport layers on the fly, but the
SSL vs quic part might be tricky to handle.