* gnus-art.el (gnus-article-setup-buffer): Go to summary buffer first to test

* gnus-art.el (gnus-article-setup-buffer): Go to summary buffer first to test
 gnus-single-article-buffer which may be buffer-local.

* gnus-sum.el (gnus-summary-setup-buffer): Make gnus-single-article-buffer
 buffer-local and nil in ephemeral group;
 make gnus-article-buffer, gnus-article-current, and
 gnus-original-article-buffer always buffer-local.
(gnus-summary-exit): Kill article buffer belonging to ephemeral group.
(gnus-handle-ephemeral-exit): Don't move to next summary line.