Chunnel: Increase tunnel pool size, make tunnel IDs 32 bit.

Add lookup mechanism for tunnel IDs which don't fit in 16 bits in M2.

src/saveload/tunnel_sl.cpp

@@ -41,6 +41,7 @@ static void Load_TUNN()
 	while ((index = SlIterateArray()) != -1) {
 		Tunnel *tunnel = new (index) Tunnel();
 		SlObject(tunnel, _tunnel_desc);
+		tunnel->UpdateIndexes();
 	}
 }