* lisp/gnus/message.el: Reduce redundancy with send-mail-function

* lisp/gnus/message.el: Reduce redundancy with send-mail-function

(message-send-mail-function) <function>: Remove `local-library` tests
for libs distributed with Emacs.
(message-use-send-mail-function): New function.
(message-default-send-mail-function): Default to it, and remove cases
already handled by it.
(message--default-send-mail-function): New function.
(message-send-mail-function) <variable>: Use it as new default.
(message-sendmail-f-is-evil): Obey mail-specify-envelope-from if available.
(message-check, message-with-reply-buffer): Use `declare`.
(message-smtpmail-send-it): smtpmail accepts mail-header-separator,
so simplify and declare obsolete.
(message-send-mail-with-mailclient): Declare obsolete.
(message-check-news-body-syntax): Don't presume that the checksum is
a fixnum.