(gnus-decode-header-function): Use

(gnus-decode-header-function): Use
`mime-decode-header-in-region'.
(article-decode-charset): Correspondence with FLIM.
(gnus-article-prepare): Likewise.
(gnus-article-prepare-display): New optional argument.
(gnus-mime-view-part-as-type): Rename `mailcap-*' to `mm-mailcap-*'.
(gnus-display-mime): New required argument. Correspondence with FLIM.
(gnus-mime-display-part): Likewise.
(gnus-mime-display-single): Likewise.
(gnus-mime-display-alternative): Likewise.
(gnus-mime-display-alternative): Likewise.