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=""
```
assert(util::SettingsSpan(*includes).last_negated());
                use_conf_file = false;
            }
        }
        if (use_conf_file) {
            std::string chain_id = GetChainName();
            std::vector<std::string> conf_file_names;
            auto add_includes = [&](const std::string& network, size_t skip = 0) {
                size_t num_values = 0;
                LOCK(cs_args);
                if (auto* section = util::FindKey(m_settings.ro_config, network)) {
                    if (auto* values = util::FindKey(*section, "includeconf")) {
                        for (size_t i = std::max(skip, util::SettingsSpan(*values).negated()); i < values->size(); ++i) {
                            conf_file_names.push_back((*values)[i].get_str());
                        }
                        num_values = values->size();
                    }
                }
                return num_values;
            };
            
            
            const size_t chain_includes = add_includes(chain_id);
            const size_t default_includes = add_includes({});
            for (const std::string& conf_file_name : conf_file_names) {
                fsbridge::ifstream conf_file_stream(GetConfigFile(conf_file_name));
                if (conf_file_stream.good()) {
                    if (!ReadConfigStream(conf_file_stream, conf_file_name, error, ignore_invalid_keys)) {
                        return false;