Add optional fourth argument, return number of appended message if possible

Add optional fourth argument, return number of appended message if possible

* elmo.el (elmo-folder-append-buffer): Add optional fourth
  argument, return number of appended message if possible.
* elmo-pipe.el (elmo-folder-append-buffer): Dto.
* elmo-maildir.el (elmo-folder-append-buffer): Dto.
* elmo-flag.el (elmo-folder-append-buffer): Dto.
* elmo-localdir.el (elmo-folder-append-buffer): Dto.
* elmo-filter.el (elmo-folder-append-buffer): Dto.
* elmo-cache.el (elmo-folder-append-buffer): Dto.
* elmo-archive.el (elmo-folder-append-buffer)
(elmo-archive-folder-append-buffer): Dto.