(message-required-news-headers): `X-Newsreader' was replaced by `User-Agent'.

(message-required-news-headers): `X-Newsreader' was replaced by `User-Agent'.
(message-required-mail-headers): `X-Mailer' was replaced by `User-Agent'.
(message-header-format-alist): `X-Mailer' and `X-Newsreader' were
replaced by `User-Agent'.
(message-generate-headers): Ditto.

(message-user-agent): New variable.
(message-newsreader): Replaced by `message-user-agent'.
(message-mailer): Ditto.
(message-mode): `message-mailer' and `message-newsreader' were replaced
by `message-user-agent'.

(message-fill-address): Use SPACE instead of HTAB.
(message-fill-header): Ditto.
(message-fill-references): Removed.

(message-use-mail-copies-to): New variable.
(message-use-mail-followup-to): Modify doc-string.
(message-use-mail-reply-to): Ditto.
(message-goto-mail-copies-to): New function.

(message-reply): New implementation.
(message-followup): Ditto.