* elmo.el (elmo-folder-writable-p): Changed default value to nil.

* elmo.el (elmo-folder-writable-p): Changed default value to nil.
(elmo-folder-move-messages): Check `elmo-folder-writable-p'.

* elmo-archive.el (elmo-folder-writable-p): Defined.

* elmo-cache.el (elmo-folder-writable-p): Ditto.

* elmo-filter.el (elmo-folder-writable-p): Ditto.

* elmo-imap4.el (elmo-folder-writable-p): Ditto.

* elmo-localdir.el (elmo-folder-writable-p): Ditto.

* elmo-maildir.el (elmo-folder-writable-p): Ditto.

* elmo-mark.el (elmo-folder-writable-p): Ditto.

* elmo-pipe.el (elmo-folder-writable-p): Ditto.

* elmo-nntp.el (elmo-folder-writable-p): Eliminated.