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/Activities/FindAndDeliverResources.cs

@@ -191,7 +191,7 @@ namespace OpenRA.Mods.Common.Activities
 				.WithCustomCost(loc =>
 				{
 					if ((loc - searchFromLoc).LengthSquared > searchRadiusSquared)
-						return PathGraph.CostForInvalidCell;
+						return PathGraph.PathCostForInvalidPath;
 
 					// Add a cost modifier to harvestable cells to prefer resources that are closer to the refinery.
 					// This reduces the tendancy for harvesters to move in straight lines