cdcleanup(). This fixes sysctl problems ("can't re-use a leaf") when
someone adds another peripheral at the same unit number. (e.g. rescan da0,
it goes away, then rescan again and da0 comes back, but since we haven't
cleaned up the sysctl variables from the last da0 instance, we can't
register the variables for the new instance under the same name.)
Reported by: njl
Tested by: njl
70bd0898d6