(mime-preview-condition): Specify 'childrens-situation field for

(mime-preview-condition): Specify 'childrens-situation field for
message/rfc822 and message/news.
(mime-view-display-message): Use value of 'childrens-situation field
of preview-situation as default-situation of children.
(mime-view-display-entity): Add new argument 'default-situation; use
it as elements of draft of preview-situation; use value of 'header
field of preview-situation instead of 'mime-view-header-visible-p; use
value of 'childrens-situation field of preview-situation as
default-situation of children.