(make-mime-entity): Change format.

(make-mime-entity): Change format.
(mime-entity-header-start): New function.
(mime-entity-header-end): New function.
(mime-entity-body-start): New function.
(mime-entity-body-end): New function.
(mime-entity-content-type): Modify for new format.
(mime-entity-content-disposition): Modify for new format.
(mime-entity-encoding): Modify for new format.
(mime-entity-children): Modify for new format.
(mime-entity-point-min): Change to alias of 'mime-entity-header-start.
(mime-entity-point-max): Change to alias of 'mime-entity-body-end.
(mime-parse-multipart): Modify for 'make-mime-entity.
(mime-parse-message): Modify for 'make-mime-entity.