* gnus-sum.el (gnus-summary-maybe-hide-threads): Use predicate

* gnus-sum.el (gnus-summary-maybe-hide-threads): Use predicate
function.
(gnus-article-unread-p): New function.
(gnus-article-unseen-p): New function.
(gnus-dead-summary-mode-map): Typo.

* gnus-util.el (gnus-make-predicate): New function.
(gnus-make-predicate-1): New function.

* gnus-sum.el: New function.
(gnus-map-articles): New function.

* gnus-art.el (gnus-treat-fold-headers): New variable.
(gnus-article-treat-fold-headers): New command and keystroke.

* gnus-sum.el (gnus-dead-summary-mode-map): Clean up.
(gnus-dead-summary-mode-map): Bind q to bury-buffer.