Update to 2.23

Upstream changes:
2.23  Wed Oct 14 09:11:00 2015
- Add the utf8 BOM to this file. Let's hope the CPAN toolchain can cope. If it can't, another
version of the code will have to be released.
- Fix read() and write() so they work on files called '0' (zero). See RT#107754.
Many thanx to Григорий Кудренко for the report and patch.
- Add t/05.zero.t and t/0 to test the new code.
- Reformat the source slightly.

(wen)