(nnheader-file-name-translation-alist): Modify DOC-string to sync with

(nnheader-file-name-translation-alist): Modify DOC-string to sync with
Pterodactyl Gnus v0.66.
(nnheader-nov-read-integer): Use `char-after' instead of
`following-char' <to sync with Pterodactyl Gnus v0.66>; use
`(search-forward "\t" eol 'move)' instead of `(forward-char 1)' as
same as `gnus-nov-read-integer'.
(nnheader-parse-nov): Use `char-after' instead of `following-char' <to
sync with Pterodactyl Gnus v0.66>.
(nnheader-retrieve-headers-from-directory*): New function.
(nnheader-retrieve-headers-from-directory): Use
`nnheader-retrieve-headers-from-directory*'.
(nnheader-get-newsgroup-headers-xover*): New function.
(nnheader-insert-references): Modify comment to sync with Pterodactyl
Gnus v0.66.