(rmail-convert-babyl-to-mbox): Disable undo while

(rmail-convert-babyl-to-mbox): Disable undo while
inserting the converted mailbox.
(rmail-get-new-mail): Restore the loop over inboxes when two or more
have the same basename.  (Bug#2164)
Set the return value according to whether any mail was found,
not just in the last inbox.  Only show a message and run the hook after
checking all inboxes.
(rmail-get-new-mail-1): Move message display and running the hook to
rmail-get-new-mail.