Fix Copyright, keywords, whitespace, etc.

Fix Copyright, keywords, whitespace, etc.
(gnus-bookmark-default-file): Use gnus-directory.
(gnus-bookmark-bmenu-file-column, gnus-bookmark-use-annotations):
Remove "*" in doc string.
(gnus-bookmark-write-file): Simplify.
(gnus-bookmark-maybe-sort-alist): Use `when'.
(gnus-bookmark-get-bookmark): Fix typo in doc string.
(gnus-bookmark-set-bookmark-name, gnus-bookmark-get-bookmark): Add
FIXME about Emacs 21 and XEmacs compatibility.
(gnus-bookmark-set-bookmark-name): Use `gnus-replace-in-string' for
compatibility.
(gnus-bookmark-bmenu-mode): Use `gnus-run-mode-hooks' for
compatibility.