multiplayer stage 1: --host/--connect startup, client without a local scenario

Networking used to be reachable only through the eu07.ini network.server /
network.client entries, and init_modes() refused to start at all unless a
scenery file was already known. That made it impossible to join a session
without knowing (and typing) the host's scenario up front, even though
SERVER_HELLO already carries the scenario name.

- add --host [address:port] and --connect address:port to the command line
  (default port 7420, --host defaults to 0.0.0.0), plus --scenario/--vehicle
  aliases and -h/--help; the existing ini settings keep working
- drop the init_modes() restriction: a client goes straight to the scenario
  loader and waits there for the handshake, a host without -s still gets the
  launcher and simply refuses joins until a scenario is up
- the loader screen reports the network state instead of sitting on a blank
  progress bar
- a client that did not ask for a vehicle with -v starts as an observer
  rather than failing to find the default EU07-424; the lobby of stage 2 will
  take over the choice
- handshake carries build identification and the server answers an
  impossible join with SERVER_REJECT and a readable reason instead of
  dropping the socket silently; protocol version bumped to 3
- deserialize_message() no longer dereferences a null message on an unknown
  message type

utilities/Globals.h

@@ -81,6 +81,9 @@ struct global_settings {
     std::string szDefaultExt{ szTexturesDDS };
 	std::string SceneryFile;
     std::string local_start_vehicle{ "EU07-424" };
+    // set when the starting vehicle was explicitly requested (command line -v),
+    // so multiplayer clients can tell an intentional choice from the built-in default
+    bool local_start_vehicle_override{ false };
     int iConvertModels{ 0 }; // tworzenie plików binarnych
     int iConvertIndexRange{ 1000 }; // range of duplicate vertex scan
     bool file_binary_terrain{ true }; // enable binary terrain (de)serialization
@@ -347,6 +350,10 @@ struct global_settings {
 	std::vector<std::pair<std::string, std::string>> network_servers;
 	std::optional<std::pair<std::string, std::string>> network_client;
 	float desync = 0.0f;
+	// human readable state of the network session, presented on the connecting/loading screen
+	std::string network_status;
+	// set when the server refused us; keeps the reason for the ui and stops reconnect attempts
+	std::string network_reject_reason;
 
 	std::unordered_map<int, std::string> trainset_overrides;