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

utilities/Globals.h

@@ -372,6 +372,9 @@ struct global_settings {
 	uint64_t network_session_token = 0;
 	// how far the worst vehicle was out of place when the last correction arrived, metres
 	float network_position_error = 0.0f;
+	// the scenario has finished loading and the world can be worked with. this is not the
+	// same as simulation::is_ready, which only goes up once the player has a cab
+	bool simulation_loaded = false;
 
 	std::unordered_map<int, std::string> trainset_overrides;