The following patches attempt to provide a simple, extendable approach
to handling the 'Seen' maildir flag. To appease (hopefully) everyone, it
will only do this for new messages. This means that people coming from
another MUA won't be stuck with 30,000 unread messages, for example.

It should be simple to extend this to other maildir flags, too, if
people want them and can decide on what tags they should correspond to.