multiplayer stage 2: peer identity, stable vehicle ids and a session lobby

TTrain::id() numbers cabs with a local counter, in the order each peer happens
to create them, so it cannot address a vehicle across the session. Neither was
there any way for a player to see what is available to drive.

- network/entities: PeerId (the host is peer 1, remote peers are numbered from
  2, so it is a participant rather than a special case) and NetworkEntityId,
  plus a registry that numbers the drivable vehicles of the loaded scenario in
  name order and resolves ids back to TDynamicObject
- the server owns the numbering and publishes it as VEHICLE_LIST - name, crew
  occupancy, crew capacity, AI state, availability - on every change and at
  least every 120 frames, out of band so it stays out of the replay backbuffer;
  a client binds the received ids to its own copy of the scenario by name
- SERVER_HELLO hands the joining peer its PeerId
- a multiplayer lobby listing that roster, with vehicles taken by other players
  shown as such rather than hidden; entering one goes through the existing
  entervehicle command, the vehicle is never moved

The lobby lives inside the driver mode instead of a mode of its own: the world
has to keep running while somebody is choosing (a client stops consuming the
server's frames the moment its mode is not a command processor), and the same
screen is what a player needs to change vehicles later on.

Crew occupancy is still derived from the presence of a cab instance; the real
crew model, claim handshake and AI takeover follow in stage 3.

network/manager.cpp

@@ -59,11 +59,37 @@ network::manager::manager()
 {
 }
 
+void network::server_manager::publish_vehicle_list()
+{
+	if (Entities.empty())
+		return;
+
+	Entities.refresh();
+
+	// resend on every change, and once in a while regardless, so that a peer which has
+	// just finished catching up does not have to wait for somebody to move
+	bool const changed = (Entities.entries() != last_published_list);
+	if (!changed && --publish_countdown > 0)
+		return;
+
+	last_published_list = Entities.entries();
+	publish_countdown = PUBLISH_INTERVAL_FRAMES;
+
+	vehicle_list msg;
+	msg.vehicles = last_published_list;
+
+	for (auto srv : servers)
+		srv->push_message(msg);
+}
+
 void network::manager::update()
 {
 	for (auto &backend : backend_list())
 		backend.second->update();
 
+	if (servers && simulation::is_ready)
+		servers->publish_vehicle_list();
+
 	if (client)
 		client->update();
 }