Add a new flag, '-C' which enables a special mode that is intended for
catastrophic recovery. Currently, this mode only validates whether a
cylindergroup has good signature data, and prompts the user to decide
whether to clear it as a whole.

This mode is useful when there is data damage on a disk and you are
working on copy of the original disk, as fsck_ffs(8) tends to abnormally
exit in such case, as a last resort to recover data from the disk.