Some things are probably broken when we advertise multiple ORPorts and only some are reachable

Observations on reachability testing made while fixing #12160 (moved):

• We only have a 1-bit notion of reachability; if we get an incoming non-local connection, we assume reachability in onionskin_answer() and call router_orport_found_reachable() to publish a descriptor.

• We should have a reachability bit per advertised ORPort to determine its inclusion in the published descriptor, and publish if and only if we have one or more reachable ORPorts.

• To implement this, we need a way to link incoming testing circuits to a particular advertised ORPort; we don't know this from the port the underlying channel was listening on because reverse proxies might make this not one-to-one in general.

• Arma suggests in IRC that netinfo cells know the IP the connection was attempted on and if they were extended with a port number they might provide a sufficient mechanism.