Bug in `nodecheck` disk checker.

Back around FreeBSD 9, the "ad" SATA devices got renamed as "ada" devices. For compatibility, the old /dev device was left around. The old mapping seemed to always be (old -> new): ad4 -> ada0, ad6 -> ada1, ad8 -> ada2, etc.

We have a number of scripts which still use the old names and we map them according to that formula. Unfortunately, the new NUC node doesn't adhere to that convention. It says:

ada0: Previously was known as ad6

which breaks the diskcheck script:

Running Hardware Inventory Gather
  Gathering Inventory.. Starting diskcheck../etc/testbed/diskcheck: line 685: ad6: unbound variable
Done Running Hardware Inventory

And as a related heads-up, starting in FreeBSD 11, the compatibility devices are gone and you have to work with the "ada" names.