Fix bug #9520 with MIME display toggling.

Fix bug #9520 with MIME display toggling.

 lisp/mail/rmailmm.el (rmail-mime-display-header)
 (rmail-mime-display-tagline, rmail-mime-display-body): New defsubsts.
 (rmail-mime-entity-segment, rmail-mime-toggle-raw)
 (rmail-mime-toggle-hidden, rmail-mime-insert-text)
 (rmail-mime-insert-bulk, rmail-mime-insert-multipart)
 (rmail-mime-insert, rmail-mime-insert-tagline): Use them instead
 of a raw aref.
 (rmail-mime-entity-segment): To get past the tagline, move forward
 2 more lines, to account for the 2 empty lines that precede and
 follow the line with the buttons.
 (rmail-mime-update-tagline): Move one more line, to get past the
 empty line that follows the buttons in the tagline.