Stop small buildings from spawning on highways.

+Changed it so buildings like LMOE shelters and Labs use "wilderness" spawn type instead of "land" spawn type, since "land" includes roads and other buildings. "land" spawns should be restricted to things happen after the apocalypse, like craters and slime pits. 
+Changed place_special in omapgen to happen after road placement, to reduce the number of unnecessary roads. 
+Reduced the amount of swamps generated so the rivers banked by solid swampland.