(sendmail-send-it): Reenaable the code

(sendmail-send-it): Reenaable the code
to compute resend-to-address and use it.

(mail-yank-ignored-headers)
(mail-font-lock-keywords, mail-mode-fill-paragraph):
Add Mail-Followup-To and Mail-Reply-To headers.
(mail-citation-hook): Add autoload cookie.
(mail-mode): Doc fix.
(mail-mode-map): Bind mail-mail-followup-to and mail-mail-reply-to.
(mail-send): Compute Mail-Followup-To and Mail-Reply-To headers.
(mail-mode-fill-paragraph): Handle those headers.
(mail-mailing-lists): New variable.
(mail-mail-reply-to, mail-mail-followup-to): New functions.