Mail::Message::Subject

fix copyright.

private-ify: psw_*() -> _psw_*().

use article_summary cache as coudl as possible. To clean up around the
codes, some codes are separeted to _generate_summary().

fix comments and documents.

strict check of $status code.

implement get_overview($id, $thread_args). It returns string holding
article short summaries of a thread for this article $id.

$curproc->thread_db_args(); -> $curproc->article_thread_init();