Attached is the latest imap-db.el.  It only does file-exists-p right
now, but it's got the functions for file retrieval and file creation
in place.  It can't write over an existing file yet (replace body).
Plus, I think the file retrieval may need to get just the body, not
the body plus the headers.

imap-db-create-article is especially hairy.  I'm not sure I'm doing
the right things in there.  I'm not inserting the new article in the
nnmail cache, for instance.

I added the imap-possibly-change-group function, feel free to move it
to imap.el if it's OK.

I picked a fairly simple format:

nnimap://mail.lifelogs.com/ding?SUBJECT%20spam

that's it, no UIDs, no authentications, no uidvalidity.  I'm happy to
let nnimap.el handle all of that.

Anyhow, let me know if there are obvious bugs or errors in the code.

Ted