Resurrect: get the basic signature matching code working again.

Enhance: add -U option to have imagezip update (or create) the
signature file. Previously, the signature file was created off-line
on boss with the imagehash command (that would be Mike's "imagehash"
(/usr/testbed/bin/imagehash) and not Leigh's "imagehash"
(/usr/testbed/sbin/imagehash)). Creating it as we create the image
makes a lot of sense...except for how we do it. We actually read and
create the hashes as a separate pass before we re-read, compress, and
create the image--so we read the disk twice. [This is primarily because we
are mooching off of the existing hash checking code (-H option). Doing
this right will require re-writing The Big Loop which makes a single
pass through the data, simultaneously dealing with disk IO, allocated
ranges, and compression blocks all of which have different size/alignment
criteria. But I digress...] Anyway, reading the disk data twic...