(gnus-treat-predicate): Handle the new treatment variables `mime' and `nomime'.

(gnus-treat-predicate): Handle the new treatment variables `mime' and `nomime'.
(gnus-article-decode-message-body-as-default-mime-charset): New function.
(gnus-article-prepare-display): Rewrite for the use of `gnus-treat-article'.
(gnus-article-prepare-mime-display): New function.
(article-date-ut): Use `next-single-property-change' instead of re-search.
(gnus-treatment-function-alist): Add a pair of
`gnus-treat-decode-message-body-as-default-mime-charset' and
`gnus-article-decode-message-body-as-default-mime-charset'.
(gnus-treat-decode-message-body-as-default-mime-charset): New user option.