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(cmd.find('=') == std::string::npos);
    Assert(cmd.at(0) != '-');
    LOCK(cs_args);
    m_accept_any_command = false; 
    std::map<std::string, Arg>& arg_map = m_available_args[OptionsCategory::COMMANDS];
    auto ret = arg_map.emplace(cmd, Arg{"", help, ArgsManager::COMMAND});
    Assert(ret.second); 
}
void ArgsManager::AddArg(const std::string& name, const std::string& help, unsigned int flags, const OptionsCategory& cat)
{
    Assert((flags & ArgsManager::COMMAND) == 0); 
    
    size_t eq_index = name.find('=');
    if (eq_index == std::string::npos) {
        eq_index = name.size();
    }
    std::string arg_name = name.substr(0, eq_index);
    LOCK(cs_args);
    std::map<std::string, Arg>& arg_map = m_available_args[cat];
    auto ret = arg_map.emplace(arg_name, Arg{name.substr(eq_index, name.size() - eq_index), help, flags});
    assert(ret.second); 
    if (flags & ArgsManager::NETWORK_ONLY) {
        m_network_only_args.emplace(arg_name);
    }
}
void ArgsManager::AddHiddenArgs(const std::vector<std::string>& names)