(gnus-treat-predicate): Abolish the treatment variable `nomime'.

(gnus-treat-predicate): Abolish the treatment variable `nomime'.
(gnus-article-prepare-mime-display): Call `gnus-treat-article' with the 1st arg
t for treating if the part is not the last one.
(gnus-article-decode-article-as-default-mime-charset): Renamed from
`gnus-article-decode-message-body-as-default-mime-charset'; Don't work if
`gnus-show-mime' is non-nil.
(gnus-treat-decode-article-as-default-mime-charset): Renamed from
`gnus-treat-decode-message-body-as-default-mime-charset'; add choice item for
decoding the whole article.