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=""
```
{
    const util::SettingsValue value = GetSetting(strArg);
    return value.isNull() ? nDefault : value.isFalse() ? 0 : value.isTrue() ? 1 : value.isNum() ? value.get_int64() : atoi64(value.get_str());
}
bool ArgsManager::GetBoolArg(const std::string& strArg, bool fDefault) const
{
    const util::SettingsValue value = GetSetting(strArg);
    return value.isNull() ? fDefault : value.isBool() ? value.get_bool() : InterpretBool(value.get_str());
}
bool ArgsManager::SoftSetArg(const std::string& strArg, const std::string& strValue)
{
    LOCK(cs_args);
    if (IsArgSet(strArg)) return false;
    ForceSetArg(strArg, strValue);
    return true;
}
bool ArgsManager::SoftSetBoolArg(const std::string& strArg, bool fValue)
{
    if (fValue)
        return SoftSetArg(strArg, std::string("1"));
    else
        return SoftSetArg(strArg, std::string("0"));
}
void ArgsManager::ForceSetArg(const std::string& strArg, const std::string& strValue)
{
    LOCK(cs_args);
    m_settings.forced_settings[SettingName(strArg)] = strValue;
}
void ArgsManager::AddCommand(const std::string& cmd, const std::string& help)