hardlink: add --list-duplicates and --zero

hardlink: add --list-duplicates and --zero

--list-duplicates codifies what everyone keeps re-implementing with
find -exec b2sum or src:perforate's finddup or whatever.

hardlink already knows this, so make the data available thusly,
in a format well-suited for pipeline processing
(fixed-width key for uniq/cut/&c.,
 tab delimiter for cut &a.,
 -z for correct filename handling).