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.

application/drivermode.cpp

@@ -266,6 +266,14 @@ bool driver_mode::update()
 
 		// variable step simulation time routines
 
+		if (change_train.empty() && !Global.network_pending_vehicle.empty())
+		{
+			// vehicle picked in the multiplayer lobby; the cab itself is built by the
+			// replicated entervehicle command, we only have to move in once it exists
+			change_train = Global.network_pending_vehicle;
+			Global.network_pending_vehicle.clear();
+		}
+
 		if (!change_train.empty())
 		{
 			TTrain *train = simulation::Trains.find(change_train);
@@ -452,6 +460,15 @@ void driver_mode::enter()
 		Error("Bad scenario: failed to locate player train, \"" + Global.local_start_vehicle + "\"");
 	}
 
+	if (Application.is_server() || Application.is_client())
+	{
+		// in a multiplayer session nobody is dropped into a random vehicle: the roster
+		// published by the server is offered instead, and stays available for later switches
+		auto ui = std::dynamic_pointer_cast<driver_ui>(m_userinterface);
+		if (ui != nullptr)
+			ui->show_multiplayer_lobby(nPlayerTrain == nullptr);
+	}
+
 	// if (!Global.bMultiplayer) //na razie włączone
 	{ // eventy aktywowane z klawiatury tylko dla jednego użytkownika
 		KeyEvents[0] = simulation::Events.FindEvent("keyctrl00");