multiplayer: fix the black screen - a cab and is_ready waiting on each other

The renderer draws nothing at all while simulation::is_ready is false, and that
flag only goes up once the local player's cab is built.

The previous commit made walking into a cab a crew request, so the cab of a
claimed vehicle is created by Crews::reconcile() rather than by the command
itself. reconcile() runs from the network housekeeping, which was gated on
simulation::is_ready - so a host started with -v had its entervehicle turned
into a claim, and then the cab waited for is_ready while is_ready waited for
the cab. Black screen on both ends, since a client whose server never gets
going has nothing to show either.

- the housekeeping now waits for Global.simulation_loaded, which says the world
  exists, instead of simulation::is_ready, which says the player has a cab
- in a session, a player with no cab counts as ready: they are an observer in
  the lobby, not a game still loading, and the world has to be drawn for them
- entervehicle is only taken as a crew request when the session actually lists
  that vehicle; anything else stays an ordinary command, so a scenario that
  starts its host in something the roster does not cover still works

network/session.cpp

@@ -120,10 +120,12 @@ void filter_commands(PeerId Peer, command_queue::commands_map &Commands)
 				continue;
 			}
 
-			if (command->command == user_command::entervehicle)
+			if ((command->command == user_command::entervehicle) && (Entities.id_of(command->payload) != ENTITY_NONE))
 			{
 				// the local player walked into a cab. that is a crew request; the cab
-				// itself is built by the authority once the seat is granted
+				// itself is built by the authority once the seat is granted.
+				// a vehicle the session does not list falls through to the ordinary path,
+				// so a scenario that starts its host in something unusual still works
 				NetworkEntityId entity{ENTITY_NONE};
 				auto const result = claim_by_name(Peer, command->payload, entity);
 				if (result != claim_result::granted && result != claim_result::already_member)