Dungeons: Add nodebox stairs to desert and sandstone dungeons

Desert and sandstone dungeons have 2 node wide corridors. Previously,
nodebox stairs were disabled because dungeon generation code did not
support nodebox stairs wider than 1 node, now it does.

Add 'stair desert stone' content id to MappgenBasic.

Requires 'mapgen stair desert stone' to be added to Minetest Game.

src/mapgen.h

@@ -266,6 +266,7 @@ class MapgenBasic : public Mapgen {
 	content_t c_cobble;
 	content_t c_stair_cobble;
 	content_t c_mossycobble;
+	content_t c_stair_desert_stone;
 	content_t c_sandstonebrick;
 	content_t c_stair_sandstonebrick;