Don't query before visiting a Babyl file.

Don't query before visiting a Babyl file.
Instead, query before modifications that would save it.

* mail/rmail.el (rmail-was-converted): New variable.
(rmail-seriously-modified): New variable.
(rmail-perm-variables): Set them to nil.
(rmail-convert-babyl-to-mbox): Don't query about converting Babyl file.
Instead, set rmail-was-converted.
(rmail-modify-format): New function.  Query if save will change format.
(rmail-dont-modify-format): New function.
(rmail-duplicate-message, rmail-get-new-mail):
Call rmail-modify-format.
(rmail-set-header): Call rmail-dont-modify-format.
(rmail-set-attribute): Likewise.
(rmail-expunge-confirmed): Call rmail-modify-format.

* mail/rmailout.el (rmail-output-to-rmail-buffer):
Call rmail-modify-format.