When sep='/' in a Maildir, we were doing a os.path.join(dirname,'') on
the top level maildir, which results in a "dirname/", so all our maildir
folder names had slashes appended. Which is pretty much wrong, so this
fixes it by only using os.path.join when we actually have something to
append.
Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>