Since Windows Vista, locale handling is converted from using numeric
locale identifiers (LCID) to using ISO5646 locale strings. In the
meantime Windows introduced new locales which don't even have a LCID
attached. Those were unusable in Cygwin because locale information
for these locales required to call the new locale functions taking
a locale string.
Convert Cygwin to drop LCIDs and use Windows ISO5646 locales instead.
The last place using LCIDs is the __set_charset_from_locale function.
Checking numerically is easier and uslay faster than checking strings.
However, this function is clearly a TODO
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