Introduce an *empty* debug type

This debug type will always be enabled whenever any debugging is enables
and it outputs debug messages that cannot be categorized among any of
imap, maildir (e.g. things that concern the sync logic).

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/init.py

@@ -222,7 +222,8 @@ class OfflineImap:
                 ui._msg("Debug mode: Forcing to singlethreaded.")
                 options.singlethreaded = True
 
-            for type in options.debugtype.split(','):
+            debugtypes = options.debugtype.split(',') + ['']
+            for type in debugtypes:
                 type = type.strip()
                 ui.add_debug(type)
                 if type.lower() == 'imap':