Dungeons: Settable density noise, move number calculation to mapgens (#8473)

Add user-settable noise parameters for dungeon density to each mapgen,
except V6 which hardcodes this noise parameter.

Move the calculation of number of dungeons generated in a mapchunk out
of dungeongen.cpp and into mapgen code, to allow mapgens to generate
any desired number of dungeons in a mapchunk, instead of being forced
to have number of dungeons determined by a density noise.

This is more flexible and allows mapgens to use dungeon generation to
create custom structures, such as occasional mega-dungeons.

src/mapgen/mapgen_flat.h

@@ -48,6 +48,7 @@ struct MapgenFlatParams : public MapgenParams
 	NoiseParams np_filler_depth;
 	NoiseParams np_cave1;
 	NoiseParams np_cave2;
+	NoiseParams np_dungeons;
 
 	MapgenFlatParams();
 	~MapgenFlatParams() = default;