When walking the body of a message/rfc822 body to insert the latter

When walking the body of a message/rfc822 body to insert the latter
in-line, add BODY to the selector to disambiguate the message/rfc822
body from its own body.  MIME selectors as used by the front end
continue to diverge from IMAP sections, which appear to make little
sense (although this requires further testing with more IMAP server
implementations).