vmm.cfg: renamed maildir.folders to mailbox.folders. maildir.name
was removed. new: mailbox.format, in order to support all mailbox formats
from Dovecot. Maildir is the default format.
Adjusted VirtualMailManager/Config and update_config.py to the changes
mentioned above.
VMM/maillocation: MailLocation.__init__(): take a 'format' name,
instead of a 'directory' name.
- added function known_format() to the module.
VMM/Account: Adjusted to above changes.