(svn r15541) -Revert (r15399): 'v->cargo_type' is also used in other places, which cannot accept CT_INVALID.

-Codechange: Add Engine::GetDefaultCargoType() and Engine::CanCarryCargo() and use them.
-Fix [FS#2617]: When articulated parts have no available default cargo, use the cargo type of the first part for livery selection.
-Change: To decide whether a vehicle is refittable do not test its current capacity for being zero, but always use the 'capacity property'.
  Note: The property is used unmodifed without calling CB 15/36. By setting it to a non-zero value and returning zero in the callback vehicles can be refitted to/from zero capacity for e.g. livery effects.
  Note: It is intentional that you cannot control refittability by CB 36.

src/engine_func.h

@@ -21,7 +21,6 @@ void DrawAircraftEngine(int x, int y, EngineID engine, SpriteID pal);
 
 bool IsEngineBuildable(EngineID engine, VehicleType type, CompanyID company);
 bool IsEngineRefittable(EngineID engine);
-CargoID GetEngineCargoType(EngineID engine);
 void SetCachedEngineCounts();
 void SetYearEngineAgingStops();
 void StartupOneEngine(Engine *e, Date aging_date);