(gnus-article-prepare-display): Narrow to header with trailing newline instead

(gnus-article-prepare-display): Narrow to header with trailing newline instead
of the use of `std11-narrow-to-header'.
(article-date-ut): Use `text-property-any' instead of
`next-single-property-change'.
(article-toggle-headers): Use `text-property-any' and `text-property-not-all'
instead of `get-text-property' or `next-single-property-change'; use
`gnus-hidden-properties' instead of the property `invisible' with the function
`add-text-properties' or `remove-text-properties'; use `article-hide-headers'
instead of `gnus-article-maybe-hide-headers'; re-display X-Face image under
XEmacs after sorting fields.