Adding a dummy log writer WRITER_NONE that just discards everything.

This can be handy when oen doesn't want any output but still get
access to filter functionality, including rotation; and also for
general testing.

src/types.bif

@@ -159,6 +159,7 @@ module Log;
 
 enum Writer %{
 	WRITER_DEFAULT,
+	WRITER_NONE,
 	WRITER_ASCII,
 %}