* elmo.el (elmo-message-match-condition): Use elmo-message-fetch

* elmo.el (elmo-message-match-condition): Use elmo-message-fetch
instead of insert-file-contents-as-binary.
(elmo-message-set-field): New generic function.

* elmo-localdir.el (elmo-folder-expand-msgdb-path): Follow the
behavior change on `split-string'.

* elmo-file.el (elmo-file-find): New function.
(elmo-file-command): New user option.
(elmo-file-command-argument): Ditto.
(elmo-file-fetch-max-size): Ditto.
(elmo-file-detect-format): New function.
(elmo-map-message-fetch): Use elmo-file-fetch-max-size;
Use elmo-file-detect-format.
(elmo-map-folder-list-message-locations): Don't treat directories.