Allow to reset Deleted flag when exporting messages in Rmail

Allow to reset Deleted flag when exporting messages in Rmail

* lisp/mail/rmailout.el (rmail-output-reset-deleted-flag): New
defcustom.
(rmail-output): When 'rmail-output-reset-deleted-flag' is non-nil,
reset the Deleted flag of the appended messages, and if COUNT is
greater than 1, do not ignore deleted messages.  Update the doc
string accordingly.  (Bug#31271)

* doc/emacs/rmail.texi (Rmail Output): Document
'rmail-output-reset-deleted-flag'.

* etc/NEWS: Mention the new user option.