Resurrect/enhance delta image code. Another multi-day "one hour hack"!

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 twice sucks,
but at least it is on the client and not on boss. The take away is:
don't create your images on pc600s.

Note that -U will always create a signature file for the complete disk
or partition even when you are creating a delta image (i.e., when combined
with -H).

Enhance: add "-P <pct>" option, used with -H, which tells imagezip that if
a resulting delta image would be over <pct> percent the size (where size
is number of uncompressed sectors in the image) of a full image, then just
create a full image instead. So "-P 50" says if it would be over half the
size, "-P 200" says if it was over twice the size, etc. If you always want
a delta image to be produced, use -H without -P. If you always want a full
image, don't use -H.

This is part 2 of supporting images. Part 1 is the DB and user interface
changes that Leigh is working on. Part 3 is next up and involves modifying
the image creation MFS to download and use signatures along with the
new imagezip when creating images. Stay tuned.