gnus-art.el, mm-uu.el: Misc improvements for displaying MIME parts

gnus-art.el, mm-uu.el: Misc improvements for displaying MIME parts

* gnus-art.el (gnus-mime-inline-part, gnus-mm-display-part):
Work for the last MIME part in an article.
(gnus-mime-display-single): Suppress excessive newlines between parts.

* mm-uu.el (mm-uu-dissect): Assume that separators may be accompanied
by leading or trailing newline.