Add a difficulty setting for Jebel Barkal, refs #4963.
Allow random map scripts to read the difficulty setting.

Very Easy: Only temples and civil buildings
Easy: CivicCenter, Stables, Barracks, Embassy
Medium: Walls, Fortress, ElephantStables, Tower
Hard: Garrisoned wall turrets and one citizen archer on the walls
Very Hard: 3 Archer champions per wall segment