Use named pathfinding constants.

- Rename CostForInvalidCell to PathCostForInvalidPath
- Add MovementCostForUnreachableCell
- Update usages of int.MaxValue and short.Maxvalue to use named constants where relevant.
- Update costs on ICustomMovementLayer to return short, for consistency with costs from Locomotor.
- Rename some methods to distinguish between path/movement cost.

OpenRA.Mods.Common/Traits/Harvester.cs

@@ -206,7 +206,7 @@ namespace OpenRA.Mods.Common.Traits
 
 					// Too many harvesters clogs up the refinery's delivery location:
 					if (occupancy >= Info.MaxUnloadQueue)
-						return PathGraph.CostForInvalidCell;
+						return PathGraph.PathCostForInvalidPath;
 
 					// Prefer refineries with less occupancy (multiplier is to offset distance cost):
 					return occupancy * Info.UnloadQueueCostModifier;