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_valleys.h

@@ -67,6 +67,7 @@ struct MapgenValleysParams : public MapgenParams {
 	NoiseParams np_cave1;
 	NoiseParams np_cave2;
 	NoiseParams np_cavern;
+	NoiseParams np_dungeons;
 
 	MapgenValleysParams();
 	~MapgenValleysParams() = default;