Support rendering of HTML parts in Rmail (bug #4258).

Support rendering of HTML parts in Rmail (bug #4258).

 lisp/mail/rmailmm.el (rmail-mime-process): Handle text/html
 separately from other text/ types.  Suppress tagline for
 multipart body.
 (rmail-mime-parse): Don't change visibility of tagline here.
 (rmail-mime-set-bulk-data, rmail-mime-insert-bulk):
 Handle text/html specially.
 (rmail-mime-render-html-function,rmail-mime-prefer-html): New variables.
 (rmail-mime-insert-html, rmail-mime-render-html-shr)
 (rmail-mime-render-html-lynx): New functions.
 (rmail-mime-fix-inserted-faces): New function.
 (rmail-mime-process-multipart): Find the best part to show
 following rmail-mime-prefer-html if set.
 (rmail-mime-searching): New variable.
 (rmail-search-mime-message): Bind rmail-mime-searching to
 suppress rendering while searching.