gnus-art.el: Improve MIME part functions.

gnus-art.el: Improve MIME part functions.

gnus-art.el (gnus-article-jump-to-part): Error on no part; fix prompt.
 (gnus-mime-copy-part): Check coding system, not charset.
 (gnus-mime-view-part-externally): Never remove part.
 (gnus-mime-view-part-internally): Don't remove part here.
 (gnus-article-part-wrapper): Make sure MIME tag is visible.
 (gnus-article-goto-part): Go to displayed or preferred subpart if it is multipart/alternative.

mm-decode.el (mm-display-part): Take optional arg `force'.