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

network/message.h

@@ -13,6 +13,7 @@ struct message
 		SERVER_HELLO,
 		FRAME_INFO,
 		REQUEST_COMMAND,
+		SERVER_REJECT,
 		TYPE_MAX
 	};
 
@@ -32,6 +33,20 @@ struct client_hello : public message
 
 	int32_t version;
 	uint32_t start_packet;
+	// build identification of the connecting simulator, informational only
+	std::string app_version;
+};
+
+// sent by the server instead of SERVER_HELLO when the client cannot join;
+// carries a human readable reason so the player learns what went wrong
+struct server_reject : public message
+{
+	server_reject() : message(SERVER_REJECT) {}
+
+	std::string reason;
+
+	virtual void serialize(std::ostream &stream) const override;
+	virtual void deserialize(std::istream &stream) override;
 };
 
 struct server_hello : public message
@@ -42,6 +57,8 @@ struct server_hello : public message
 	int64_t timestamp;
     int64_t config;
     std::string scenario;
+	// build identification of the server, informational only
+	std::string app_version;
 
 	virtual void serialize(std::ostream &stream) const override;
 	virtual void deserialize(std::istream &stream) override;