Modify format of paths in metadata and minor fixes.

Allow leading os.sep for target and restricted paths listed in metadata.  Previously, '{repository directory}/targets/file.txt' was listed as 'file.txt' and unsuccessfully compared against '/', or the root-most targets directory.  Now, '/file.txt' & '/' and similar examples may be compared and verified.

Fix issue where a hashed bin with empty targets was flagged as invalid because its targets did not produce a path hash prefix that matched any of those listed in the parent role.  Uncovered with _ensure_all_targets_allowed().

Minor edits to code that compared paths and and used path separators.