While testing 17270 I noticed that master (bdda137878904e9401a84e308ac74c93c2ef87c1) does not run on NetBSD. I assume this started after #17085 changed the fallback locale from `C` to `C.UTF-8`. There doesn't seem to be a `C.UTF-8` locale on NetBSD.

```bash
src/bitcoind
terminate called after throwing an instance of 'std::runtime_error'
  what():  locale::facet::_S_create_c_locale name not valid
Abort trap (core dumped)
```

`LC_ALL=C src/bitcoind` works, `LC_ALL=en_US.UTF-8 src/bitcoind` does not.

```bash
locale
LANG=""
LC_CTYPE="en_US.UTF-8"
LC_COLLATE="C"
LC_TIME="C"
LC_NUMERIC="C"
LC_MONETARY="C"
LC_MESSAGES="C"
LC_ALL=""
```
{
    fs::path path;
    if (!GetSettingsPath(&path,  false)) {
        return true; 
    }
    LOCK(cs_args);
    m_settings.rw_settings.clear();
    std::vector<std::string> read_errors;
    if (!util::ReadSettings(path, m_settings.rw_settings, read_errors)) {
        SaveErrors(read_errors, errors);
        return false;
    }
    for (const auto& setting : m_settings.rw_settings) {
        std::string section;
        std::string key = setting.first;
        (void)InterpretOption(section, key,  {}); 
        if (!GetArgFlags('-' + key)) {
            LogPrintf("Ignoring unknown rw_settings value %s\n", setting.first);
        }
    }
    return true;
}
bool ArgsManager::WriteSettingsFile(std::vector<std::string>* errors) const