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/server.cpp

@@ -266,9 +266,6 @@ Server::Server(
 	//lock environment
 	MutexAutoLock envlock(m_env_mutex);
 
-	// Load mapgen params from Settings
-	m_emerge->loadMapgenParams();
-
 	// Create the Map (loads map_meta.txt, overriding configured mapgen params)
 	ServerMap *servermap = new ServerMap(path_world, this, m_emerge);
 
@@ -331,8 +328,11 @@ Server::Server(
 
 	m_clients.setEnv(m_env);
 
+	if (!servermap->settings_mgr.makeMapgenParams())
+		FATAL_ERROR("Couldn't create any mapgen type");
+
 	// Initialize mapgens
-	m_emerge->initMapgens();
+	m_emerge->initMapgens(servermap->getMapgenParams());
 
 	m_enable_rollback_recording = g_settings->getBool("enable_rollback_recording");
 	if (m_enable_rollback_recording) {
@@ -402,11 +402,8 @@ Server::~Server()
 	m_emerge->stopThreads();
 
 	// Delete things in the reverse order of creation
-	delete m_env;
-
-	// N.B. the EmergeManager should be deleted after the Environment since Map
-	// depends on EmergeManager to write its current params to the map meta
 	delete m_emerge;
+	delete m_env;
 	delete m_rollback;
 	delete m_banmanager;
 	delete m_event;
@@ -655,7 +652,7 @@ void Server::AsyncRunStep(bool initial_step)
 					m_env->getGameTime(),
 					m_lag,
 					m_gamespec.id,
-					m_emerge->params.mg_name,
+					Mapgen::getMapgenName(m_emerge->mgparams->mgtype),
 					m_mods);
 			counter = 0.01;
 		}