* gnus-group.el (gnus-group-mark-article-read): Nreverse
gnus-newsgroups-unselected.
* gnus-agent.el (gnus-summary-set-agent-mark): Use
gnus-add-to-sorted-list.
* gnus-sum.el (gnus-summary-update-info): gnus-newsgroup-unreads
gnus-newsgroup-unselected are sorted. Use gnus-sorted-union.
(gnus-build-all-threads): Use gnus-add-to-sorted-list.
(gnus-update-read-articles): UNREAD is sorted.
(gnus-newsgroup-unreads, gnus-newsgroup-unselected)
(gnus-newsgroup-marked, gnus-newsgroup-cached)
(gnus-newsgroup-expirable, gnus-newsgroup-downloadable)
(gnus-newsgroup-dormant): Require sorted.