Prevent save-buffer in Rmail buffers from using the coding-system

Prevent save-buffer in Rmail buffers from using the coding-system
of the current message, and from clobbering the encoding mnemonics
in the mode line (Bug#4623).

(rmail-swap-buffers): Swap encoding and modified flag, too.
(rmail-message-encoding): New variable.
(rmail-write-region-annotate): Record the encoding of the current message
in rmail-message-encoding.
(rmail-after-save-hook): New function, restores the encoding of the current
message after the message collection is saved.