Add option to check signature vs. image file (rather than vs. disk)
Add option to check signature vs. image file (rather than vs. disk)

The -S (sanity check) option adds the ability to do the obvious: validate
an image file using its signature. So what did imagehash do all these years?
Well, its primary job was to generate a signature file that imagezip could
use to generate a delta. Secondarily, it would compare a signature vs. a dis
with an image loaded on it. But now we use imagezip to gen most signatures
while generating the image itself.

Also added -X (xtreme hack) option to have imagehash attempt to check or
generate signature file entries in a way that is consistent with what
imagezip does. The difference is that imagezip can produce hashes for
blocks that span chunk boundaries, imagehash (previously) never did this.

So when validating an image whose signature was generated by imagezip:

   imagehash -SX foo.ndz

To generate a new signature for an image:

   imagehash -cX foo.ndz

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