(mime-entity-situation): Don't refer `major-mode' of a

(mime-entity-situation): Don't refer `major-mode' of a
mime-entity-buffer.
(mime-display-multipart/mixed): Add `major-mode' of SITUATION to
default-situation of children.
(mime-display-multipart/alternative): Likewise.
(mime-display-entity): Don't refer `raw-buffer'; use
`mime-goto-header-start-point'.
(mime-display-message): Add new optional argument
`original-major-mode'; don't refer `raw-buffer'; use
`mime-entity-name' to make name of `preview-buffer'; don't set up
`mime-raw-buffer' of `preview-buffer'; use `original-major-mode' as
value of `major-mode' field of default-situation; don't pop up
`preview-buffer' in suitable window; return `preview-buffer'.
(mime-view-buffer): Pop up `preview-buffer' in suitable window.