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/subgame.cpp

@@ -313,8 +313,8 @@ bool loadGameConfAndInitWorld(const std::string &path, const SubgameSpec &gamesp
 		Settings conf;
 		MapgenParams params;
 
-		params.load(*g_settings);
-		params.save(conf);
+		params.readParams(g_settings);
+		params.writeParams(&conf);
 		conf.writeLines(oss);
 		oss << "[end_of_params]\n";