Refactor logging

- Add warning log level
- Change debug_log_level setting to enumeration string
- Map Irrlicht log events to MT log events
- Encapsulate log_* functions and global variables into a class, Logger
- Unify dstream with standard logging mechanism
- Unify core.debug() with standard core.log() script API

builtin/init.lua

@@ -6,7 +6,7 @@
 --
 
 -- Initialize some very basic things
-print = core.debug
+function core.debug(...) core.log(table.concat({...}, "\t")) end
 math.randomseed(os.time())
 os.setlocale("C", "numeric")
 minetest = core