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/script/lua_api/l_vmanip.cpp

@@ -190,7 +190,7 @@ int LuaVoxelManip::l_calc_lighting(lua_State *L)
 	Mapgen mg;
 	mg.vm          = vm;
 	mg.ndef        = ndef;
-	mg.water_level = emerge->params.water_level;
+	mg.water_level = emerge->mgparams->water_level;
 
 	mg.calcLighting(pmin, pmax, fpmin, fpmax, propagate_shadow);