Add support for auto-detecting when to create a delta image.
Uses the new-ish imagezip -P option. But first we had to get that option to imagezip. So the key-value interface between server and client now has an IZOPTS key where you can pass options to imagezip (This is now used to pass the -N that we have always specified as well). Also changed imagezip to print out a less ambiguous message when the option is in use, if will clearly say that it is full or delta. This matters because we have to parse the imagezip output on the server to figure out what kind of image it is!
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