* lisp/nnshimbun.el (nnshimbun-write-nov): New function.

* lisp/nnshimbun.el (nnshimbun-write-nov): New function.
(nnshimbun-close-group): Call nnshimbun-write-nov.
(nnshimbun-generate-nov-database): Ditto.
(nnshimbun-generate-nov-for-each-group): Fix bug which occur new
entries add NOV database.
(nnshimbun-generate-nov-for-all-groups): Ditto.
(nnshimbun-search-id): Add argument to return header, and modify for
search of original message id.
(nnshimbun-nov-fix-header): New function.
(nnshimbun-make-date-string): Fix for a two-digit year.