Refactor to use only Settings objects for settings.

Also remove globally stored path for modpath and data.
Basically use less global variables for better adaptation to multithreading.

terrainlib_lua/erosion.lua

@@ -123,11 +123,7 @@ end
 
 local modpath = ""
 if minetest then
-	if minetest.global_exists('mapgen_rivers') then
-		modpath = mapgen_rivers.modpath .. "terrainlib_lua/"
-	else
-		modpath = minetest.get_modpath(minetest.get_current_modname()) .. "terrainlib_lua/"
-	end
+	modpath = minetest.get_modpath(minetest.get_current_modname()) .. "terrainlib_lua/"
 end
 
 local rivermapper = dofile(modpath .. "rivermapper.lua")