Make Rmail delete and undelete commands handle repeat count.

Make Rmail delete and undelete commands handle repeat count.

* lisp/mail/rmail.el (rmail-delete-message): Update summary.
(rmail-undelete-previous-message): Handle repeat count arg.
(rmail-delete-backward, rmail-delete-forward): Likewise.

* lisp/mail/rmailsum.el (rmail-summary-delete-forward):
Optimize case of reaching end and handling count.
(rmail-summary-mark-deleted): Optimize when N is current msg.
Don't create new summary line.
(rmail-summary-undelete): Pass arg to rmail-undelete-previous-message.
(rmail-summary-undelete-many): Rewrite for speed.
(rmail-summary-msg-number): New function.