du: add -t SIZE, --threshold=SIZE option

du: add -t SIZE, --threshold=SIZE option

* src/du.c (opt_threshold): Add variable to hold the value of
the --threshold option specified by the user.
(long_options): Add a required_argument entry for the new
--threshold option.
(usage): Add --threshold option.
(process_file): Elide printing the entry if its size does not
meet the value specified by the --threshold option.
(main): In the argument parsing loop, add a case for the new
-t option. Convert the given argument by permitting the
well-known suffixes for megabyte, gigabytes, etc.
Handle the special case "-0": give an error as this value is
not permitted.
* doc/coreutils.texi (du invocation): Add documentation for the
above new option.
* tests/du/threshold.sh: Add new test to exercise the new option.
* tests/local.mk (all_tests): Mention the above test.

Co-authored-by: Bernhard Voelker <mail@bernhard-voelker.de>