makes restore less efficient, but it makes a bigger effore to read
corrupted dumps. Specifiacally, when in degreded mode:
1) Restore shifts the input by 1 byte if it sees a problem,
rather than one tape block.
2) It doesn't assume the inodes are stored in ascending order.
3) It turns some panics into warning printfs.
We also verify some fields more carefully than before.
There's probably more a degreded mode could do, but this seems to
help a lot.
Approved by: imp, iedowse, mckusick
MFC after: 3 weeks
cbc8bb98ef