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

minetest.conf.example

@@ -612,8 +612,9 @@
 #remote_media =
 
 #    Level of logging to be written to debug.txt:
-#    0 = none, 1 = errors and debug, 2 = action, 3 = info, 4 = verbose.
-#debug_log_level = 2
+#      <nothing> (no logging), none (messages with no level), error,
+#      warning, action, info, or verbose.
+#debug_log_level = action
 
 #    Maximum number of blocks that can be queued for loading
 #emergequeue_limit_total = 256