Dungeons: Use biome 'node_stone' if normal stone types not detected

Dungeons: Use biome 'node_stone' if normal stone types not detected

Construct dungeons from the node defined as biome 'node_stone' if
'mapgen_stone', 'mapgen_desert_stone' and 'mapgen_sandstone' are not
detected.
Feature long-intended by kwolekr/hmmmm and present in code as a TODO.