Patch icu to use UTF-8 instead of ASCII outside C locale and ensure UTF-8 is default

[rezny]

Summary

From PR 216372: Some QT5 (e.g. qTox) apps have trouble with non-ASCII filenames. Root cause: ucnv_getDefaultName() is called before setlocale() so it returns ASCII. The solution is to handle BSD the same as is done for Darwin and Linux; map ASCII to UTF-8 outside POSIX 'C' locale. The implementation uses the Darwin code path because it already matched in other respects (CP949 is remapped but nothing else, whereas Linux has special handling for more Asian locales). The PR has gone 2 months without action from office@

Test Plan

Poudriere 10.3/11.0 amd64/i386 OK, verified qTox now correctly handles files with Unicode names

[bapt]

please consider upstreaming that as well

[swills]

Approved