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/Pathfinder/CellInfoLayerPool.cs

@@ -25,7 +25,7 @@ namespace OpenRA.Mods.Common.Pathfinder
 		{
 			defaultLayer =
 				CellLayer<CellInfo>.CreateInstance(
-					mpos => new CellInfo(int.MaxValue, int.MaxValue, mpos.ToCPos(map), CellStatus.Unvisited),
+					mpos => new CellInfo(PathGraph.PathCostForInvalidPath, PathGraph.PathCostForInvalidPath, mpos.ToCPos(map), CellStatus.Unvisited),
 					new Size(map.MapSize.X, map.MapSize.Y),
 					map.Grid.Type);
 		}