These are changes to upasfs to add Plan B mailbox
format support.
If you don't plan to use plan 9 mail tools with Plan B
mail boxes this is not needed. You need this only if
you want imap access or using Acme's Mail with
Plan B mail boxes.

Files should go at /sys/src/cmd/upas/fs
(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 nemo@lsub.org if you need help.