When attaching MIME info records to the buffer, respect nested MIME

When attaching MIME info records to the buffer, respect nested MIME
entities so that for each point in the buffer only the most specific
MIME entity is attached.  It would be nice if there were a variant of
REGION-PUT! that layered text properties `under' existing ones, but
since there isn't, we make do by scanning the text for the property
changes and associating records for parents wherever their children
had not been attached.