(rmail): Go back to using find-file for reading in the

(rmail): Go back to using find-file for reading in the
mail file.  This avoids gratuitous modification of the file.
(rmail-decode-region): Doc string, cleanup.
(rmail-decode-by-content-type): New function.
(rmail-decode-messages): Use it.  Add FROM and TO args and only
process messages in that region.
(rmail-get-new-mail): Call `rmail-decode-region' before
`rmail-process-new-messages'.