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/World/PathFinder.cs

@@ -167,7 +167,7 @@ namespace OpenRA.Mods.Common.Traits
 				// make some progress on the first search
 				var p = fromSrc.Expand();
 				if (fromDest.Graph[p].Status == CellStatus.Closed &&
-					fromDest.Graph[p].CostSoFar < int.MaxValue)
+					fromDest.Graph[p].CostSoFar != PathGraph.PathCostForInvalidPath)
 				{
 					path = MakeBidiPath(fromSrc, fromDest, p);
 					break;
@@ -176,7 +176,7 @@ namespace OpenRA.Mods.Common.Traits
 				// make some progress on the second search
 				var q = fromDest.Expand();
 				if (fromSrc.Graph[q].Status == CellStatus.Closed &&
-					fromSrc.Graph[q].CostSoFar < int.MaxValue)
+					fromSrc.Graph[q].CostSoFar != PathGraph.PathCostForInvalidPath)
 				{
 					path = MakeBidiPath(fromSrc, fromDest, q);
 					break;