Refactoring: make functions to be private if we can

Make external API of class/module to be smaller, explicitely mark
all internal functions.  Also annotate methods that are implemented
as the part of the parent class interface.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>

offlineimap/mbnames.py

@@ -44,9 +44,9 @@ def write():
         if account not in boxes:
             return
 
-    genmbnames()
+    __genmbnames()
 
-def genmbnames():
+def __genmbnames():
     """Takes a configparser object and a boxlist, which is a list of hashes
     containing 'accountname' and 'foldername' keys."""
     mblock.acquire()