gnus.el: Fix nnmaildir marks handling

lisp/gnus/ChangeLog

 2012-02-01  Lars Ingebrigtsen  <larsi@gnus.org>
 
+	* gnus.el (gnus-valid-select-methods): nnmaildir also saves marks in
+	the "server".
+
 	* gnus-group.el (gnus-group-get-new-news-this-group): Don't overwrite
 	the real error message with the useless "previously known to be down".
 	Which isn't even correct.

lisp/gnus/gnus.el

@@ -1626,7 +1626,7 @@ slower."
     ("nnagent" post-mail)
     ("nnimap" post-mail address prompt-address physical-address respool
      server-marks)
-    ("nnmaildir" mail respool address)
+    ("nnmaildir" mail respool address server-marks)
     ("nnnil" none))
   "*An alist of valid select methods.
 The first element of each list lists should be a string with the name