(rmail-msgref-vector): New variable.

(rmail-msgref-vector): New variable.
Holds, for each message, a cons cell which contains the message number
and which relocates if the message number changes.
(rmail-forget-messages): Clear rmail-msgref-vector.
(rmail-variables): Make rmail-msgref-vector buffer-local.
(rmail-count-new-messages): Extend rmail-msgref-vector.
(rmail-set-message-counters): Initialize rmail-msgref-vector.
(rmail-only-expunge): Update rmail-msgref-vector.
Don't look for mail-mode buffers specially.
(rmail-reply): Use rmail-msgref-vector element as arg.
(rmail-forward, rmail-retry-failure): Likewise.
(rmail-mark-message): Accept an element of rmail-mark-message as arg.