Improve pathfinder support for multiple docks

The pathfinder can now find docks other than the Manhattan closest one.

Based on Cirdan commits:
afc7969c13d7ca59afe4dae4bf88122ba8d27df2
4067190dedcd3e5f668ea4f49b1dd8dfed10b2a7
d9be6b712d2ae4fb1b5ae844dde4919dd24c4fb2

src/station.cpp

@@ -313,6 +313,14 @@ Rect Station::GetCatchmentRectUsingRadius(uint catchment_radius) const
 	return ret;
 }
 
+bool Station::IsDockingTile(TileIndex tile) const
+{
+	for (const Dock *d = this->docks; d != NULL; d = d->next) {
+		if (tile == d->GetDockingTile()) return true;
+	}
+	return false;
+}
+
 /** Rect and pointer to IndustryVector */
 struct RectAndIndustryVector {
 	Rect rect;                       ///< The rectangle to search the industries in.