(rmail-msg-restore-non-pruned-header): Likewise.
If point was in the old pruned header, put it at the top.
(rmail-msg-prune-header): If point was at the top, keep it there.
(rmail-narrow-to-non-pruned-header): New function.
(rmail-retry-failure): Use rmail-narrow-to-non-pruned-header.