Buttonize MIME attachments in the article header

Buttonize MIME attachments in the article header

* gnus.texi (MIME Commands): Mention
  gnus-mime-buttonize-attachments-in-header and
  gnus-mime-display-attachment-buttons-in-header.

* gnus-art.el (gnus-mime-display-attachment-buttons-in-header):
  New user option.
  (gnus-mime-buttonize-attachments-in-header): New function.
  (gnus-article-prepare): Use it.
  (gnus-mime-inline-part): Suppress extra newline.
  (gnus-mm-display-part): Save excursion;
  remove useless deleting and adding of buttons.
  (gnus-insert-mime-button): Allow insertion in the middle of a line.

* gnus-sum.el (gnus-summary-wash-mime-map, gnus-summary-article-menu):
  Add gnus-mime-buttonize-attachments-in-header.