Re-probe 1.1.1.1 versus 1.0.0.1 if earlier probing failed

[favonia]

(Inspired by the discussion at #706.) When

1. It is meaningful to check 1.1.1.1 v.s. 1.0.0.1 and
2. All prior probing failed (we failing to probe both 1.1.1.1 and 1.0.0.1) and
3. We are about to detect potentially new IP addresses

Re-check 1.1.1.1 v.s. 1.0.0.1!