New arg RETURN-ACTION for compose-mail, and related functions.

New arg RETURN-ACTION for compose-mail, and related functions.

* lisp/mail/sendmail.el (mail-return-action): New var.
(mail-mode): Make it buffer-local.
(mail-bury): Obey it.  Move special Rmail window handling to
rmail-mail-return.
(mail, mail-setup): New arg RETURN-ACTION.
(sendmail-user-agent-compose): Move from simple.el.

* lisp/simple.el (sendmail-user-agent-compose): Move to sendmail.el.
(compose-mail): New arg RETURN-ACTION.
(compose-mail-other-window, compose-mail-other-frame): Likewise.

* lisp/gnus/gnus-msg.el (gnus-msg-mail): New arg RETURN-ACTION.  Pass it to
message-mail.

* lisp/gnus/message.el (message-mail): New arg RETURN-ACTION.
(message-return-action): New var.
(message-bury): Use it.
(message-mode): Make it buffer-local.
(message-send-and-exit): Always call message-bury.
(message-tool-bar-gnome): Tweak tool-bar items.  Add :vert-only tags.

* lisp/mail/rmail.el (rmail-mail-return): New function.
(rmail-start-mail): Pass it to compose-mail.

* lisp/mh-e/mh-comp.el (mh-user-agent-compose): New arg RETURN-ACTION.