backup disklabel for softraid(4) chunks

backup disklabel for softraid(4) chunks

Extend "Check for changes to the disklabels of mounted disks" to those that
host online softraid volumes, e.g installations with root inside CRYPTO sd0a
(and EFI System partition on sd0i).

That produces /var/backup/disklabel.sd0.current, previously missing in such
setups;  noticed after someone dd(1)ed miniroot onto sd0 by accident and had
no disklabel(8) backup to restore.

Feedback OK bluhm