Make a skip's probes drill, and count what they find

[olavgg]

A probe read 64 MiB in a second and a half and estimated what drilling its over-budget chunks would cost: fast enough that on a real drive it looked like nothing ahead was being tested, and nothing it found was counted.

• A probe drills each chunk it finds over budget with the scan's own drill-down -- every sector, slow ones retried, under the 10 s cap -- and is judged on what that actually cost, as soon as the verdict is certain.
• What it finds is counted and reported like the scan's findings, weak and bad alike, including inside a region it then condemns.
• The scan does not drill a chunk a probe already drilled, and the checkpoint keeps that list, so nothing is counted twice.