Fix a bug in fsck_ffs where if a directory somehow develops a hole
(that is a block pointer that has a value of zero), fsck would give the
filesystem a clean bill of health, but the kernel would panic when
accessing the directory with the hole. Fsck now checks for holes
in directories. If found in preen mode, fsck fails. In manual
mode, it can be directed to shorten the directory to the beginning of
the hole. A more complete solution would be to allocate a block to fill
the hole. However, this is a lot more work for a `cannot happen' error,
so the extra effort seems unwarranted.
2402aaa614