#2310 Add support for thousands digit grouping in printf() numeric formats

[Keith Marshall]

At the time when our printf() implementation -- specifically, its pformat.c component -- was written, POSIX.1-2004 was the applicable standard, and it designated the thousands digit grouping feature as an XSI extension; at that time, we made an explicit decision to withhold support for such extended features. Today, POSIX.1-2008 has become the applicable standard, and this has redesignated thousands digit grouping, together with positional argument addressing, both of which had formerly borne the XSI classification, as base standard requirements.

Having previously updated pformat.c to support positional argument addressing, (see ticket [#2280]), we should now also consider adding support for thousands digit grouping.

[Keith Marshall]

This is now implemented by commit [dbd96e], and slated for publication in mingwrt-5.0.