Add format support for efficient sparse file handling.

While making other format changes, also add in support for explicitly
representing regions of a file that are entirely zero, as can happen with
sparse files.  These are represented with an object reference of the form
"zero[0+<length>]".  Update the lbs tool to generate and parse these
references, and the utility code to also handle it.

The restore tools do not seek over zero regions when writing out a file, so
the file is not restored as a sparse file, but that support can easily be
added later with no change needed to the format.