Reduce Gnus dependencies.

Reduce Gnus dependencies.
(gnus-ems, gnus-msg, gnus-util, message, mm-decode, mml): Don't
require.  Use autoloads instead.
(mml-attach-file, mm-default-file-encoding, mailcap-extension-to-mime)
(mailcap-mime-info, mm-mailcap-command, ps-print-preprint)
(message-buffers, gnus-setup-message, gnus-print-buffer): Autoload.
(gnus-dired-mode): Adjust doc string.
(gnus-dired-mail-mode): New variable.
(gnus-dired-mode-map): Avoid using `gnus-define-keys'.
(gnus-dired-mode): Avoid using `gnus-run-hooks'.
(gnus-dired-mail-buffers): New function.  Return mail or message
composition buffers.
(gnus-dired-attach): Use it.
(gnus-dired-find-file-mailcap): Call `mailcap-mime-info' with NO-DECODE.
(gnus-dired-print): Use `gnus-print-buffer' depending on
`gnus-dired-mail-mode'.