du: General cleanup and style(9)
- Refactor duplicate threshold and print code.
- Replace obselete fts_bignum with fts_number.
- Add missing "--libxo" to usage().
- Minor style(9) fixes.
- Update list of POSIX extensions in du(1) man page.
Reviewed by: markj
MFC after: 1 month
Pull Request: https://github.com/freebsd/freebsd-src/pull/1648