nnimap -> nnmaildir, expiry settings

[Raphaël Berbain]

Hello group,

I just converted my setup from nnimap to nnmaildir.  So far, I am
quite happy with the move.  Positive effects: Mail seems to be picked
up more reliably (with nnimap talking to a local Courier server, new
mail could take a couple of minutes between delivery and Gnus seeing
it).  Also, nnmaildir has a higher start-up time cost, but behaves
faster afterwards.  Negative effects: The %m group-line-format
specifier stopped working.  Not a big deal for me, but since the %U
and %y specifier work, I wonder what could cause this.

Now, the most annoying effect is that I can't get expiry to work in
the same way.  With nnimap, once an article became eligible for
expiry, it was moved to another group and was marked read, so it was
basically left 'forgotten' there.

With nnmaildir, once an article is eligible for expiry (which means I
generally read it), it is moved to the right group.  But it then
appears unread in that group.  This is quite annoying: My expiry
groups now clutter the group buffer display - since they used to
contain no unread messages, I only saw them when I requested it with
'L', whereas now they contain unread messages, and hence appear in the
group listing.

How can I tell Gnus to mark those messages as read ?