* elmo-pop3.el (elmo-pop3-exists-exactly): Set default value as nil.

* elmo-pop3.el (elmo-pop3-exists-exactly): Set default value as nil.
(elmo-pop3-get-session): Make msgdb directory after session.
(elmo-folder-exists-p): Check exactly at the first time even when the
elmo-pop3-exists-exactly is nil.

* elmo-pipe.el (elmo-folder-open-internal): Don't drain here.
(elmo-folder-list-messages): Define instead of
elmo-folder-list-messages-internal.
(elmo-folder-check): Check destination folder.
(elmo-folder-synchronize): Drain the pipe.

* elmo-localdir.el (elmo-folder-append-messages): Don't cause an error
when msgdb is not loaded.