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/network.h

@@ -5,6 +5,7 @@
 #include <queue>
 #include <chrono>
 #include "network/message.h"
+#include "network/entities.h"
 #include "input/command.h"
 
 namespace network
@@ -47,6 +48,9 @@ namespace network
 			DEAD
 		};
 		peer_state state;
+
+		// session identity of the peer on the other side of this connection
+		PeerId peer_id { PEER_NONE };
 	};
 
 	class server
@@ -64,6 +68,9 @@ namespace network
 	public:
 		server(std::shared_ptr<std::istream> buf);
 		void push_delta(const frame_info &msg);
+		// sends an out of band message to every active peer. unlike push_delta this is not
+		// written to the backbuffer, so it does not become part of the replayed history
+		void push_message(const message &msg);
 		command_queue::commands_map pop_commands();
 	};