(gnus-summary-search-article): Rearrange.

(gnus-summary-search-article): Rearrange.
(gnus-summary-search-article-highlight-matched-text): Rearrange.
(gnus-summary-search-article-position-point): New macro.
(gnus-summary-search-article-matched-data): Rename from
`gnus-summary-search-article-matched-text'.
(gnus-summary-isearch-article): Bind `gnus-inhibit-treatment' to t;
use `gnus-article-show-all-headers' for exposing the visited article.
(gnus-summary-select-article): Bind `gnus-summary-search-article-matched-data'
in the article buffer locally.