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_v6.h

@@ -108,6 +108,8 @@ public:
 	NoiseParams *np_trees;
 	NoiseParams *np_apple_trees;
 
+	NoiseParams np_dungeons;
+
 	float freq_desert;
 	float freq_beach;
 	s16 dungeon_ymin;