Add MapSettingsManager and new mapgen setting script API functions

This commit refactors the majority of the Mapgen settings system.
- MapgenParams is now owned by MapSettingsManager, itself a part of ServerMap,
  instead of the EmergeManager.
- New Script API functions added:
    core.get_mapgen_setting
    core.get_mapgen_setting_noiseparams,
    core.set_mapgen_setting, and
    core.set_mapgen_setting_noiseparams.
- minetest.get/set_mapgen_params are deprecated by the above new functions.
- It is now possible to view and modify any arbitrary mapgen setting from a mod,
  rather than the base MapgenParams structure.
- MapgenSpecificParams has been removed.

src/CMakeLists.txt

@@ -414,6 +414,7 @@ set(common_SRCS
 	light.cpp
 	log.cpp
 	map.cpp
+	map_settings_manager.cpp
 	mapblock.cpp
 	mapgen.cpp
 	mapgen_flat.cpp