These are changes to imap4d to add Plan B mailbox
format support and to add support for the "imap.ok"
file (listing which files/dirs are ok for imap access).

If you don't plan to use imap with Plan B
mail boxes this is not needed at all.

If you want imap you must apply both this change and
also those in ../upasfs (because imap4d relies on upas/fs).

Files should go at /sys/src/cmd/ip/imap4d
(you can just bind before and compile to test it before using).

Both actual mail boxes and files with mail listings are
supported as mail boxes. This makes it easy to create virtual
folders.

Contact [email protected] if you need help.