Mgfractal: Make non-fractal terrain optional (#8702)

Enabled by default.
Only allow spawn on fractal, not on seabed terrain.
Various codestyle and comment improvements.

builtin/settingtypes.txt

@@ -1732,6 +1732,11 @@ mgflat_np_dungeons (Dungeon noise) noise_params_3d 0.9, 0.5, (500, 500, 500), 0,
 
 [*Mapgen Fractal]
 
+#    Map generation attributes specific to Mapgen flat.
+#    'terrain' enables the generation of non-fractal terrain:
+#    ocean, islands and underground.
+mgfractal_spflags (Mapgen Fractal specific flags) flags terrain terrain,noterrain
+
 #    Controls width of tunnels, a smaller value creates wider tunnels.
 mgfractal_cave_width (Cave width) float 0.09