(rmail-decode-mbox-format): Rename from `rmail-decode-mail-file'.

(rmail-decode-mbox-format): Rename from `rmail-decode-mail-file'.
(rmail-process-new-messages): Don't add missing headers here.
(rmail-convert-mbox-format): Rename from `rmail-decode-messages'.
Add missing headers here.  Remove FROM and TO arguments.
(rmail-get-new-mail): Simplify.
(rmail-convert-file): New function.
(rmail-revert): Use it.
(rmail): Change logic for avoiding selecting new messages twice.
(rmail-display-labels): Avoid space in mode-line if there are no
keywords.