bitcoind won't start if datadir is specified in the conf file and the user running it has no home dir

tested with debian stretch and bitcoin 0.17.1

```
# sudo -u bitcoin bitcoind -daemon -conf=/usr/local/etc/bitcoin/bitcoin.conf -pid=/run/bitcoind/bitcoind.pid

************************
EXCEPTION: N5boost10filesystem16filesystem_errorE
boost::filesystem::create_directories: Permission denied: "/home/bitcoin"
bitcoin in AppInit()

bitcoind: chainparamsbase.cpp:29: const CBaseChainParams& BaseParams(): Assertion `globalChainBaseParams' failed.
Aborted

# mkhomedir_helper bitcoin

# sudo -u bitcoin bitcoind -daemon -conf=/usr/local/etc/bitcoin/bitcoin.conf -pid=/run/bitcoind/bitcoind.pid
Bitcoin server starting
```


{
    for (const std::string& name : names) {
        AddArg(name, "", false, OptionsCategory::HIDDEN);
    }
}
std::string ArgsManager::GetHelpMessage() const
{
    const bool show_debug = gArgs.GetBoolArg("-help-debug", false);
    std::string usage = "";
    LOCK(cs_args);
    for (const auto& arg_map : m_available_args) {
        switch(arg_map.first) {
            case OptionsCategory::OPTIONS:
                usage += HelpMessageGroup("Options:");
                break;
            case OptionsCategory::CONNECTION:
                usage += HelpMessageGroup("Connection options:");
                break;
            case OptionsCategory::ZMQ:
                usage += HelpMessageGroup("ZeroMQ notification options:");
                break;
            case OptionsCategory::DEBUG_TEST:
                usage += HelpMessageGroup("Debugging/Testing options:");