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
```


{
    qDebug() << __func__ << ": Requesting initialize";
    startThread();
    Q_EMIT requestedInitialize();
}
void BitcoinApplication::requestShutdown()
{
    
    
    
    shutdownWindow.reset(ShutdownWindow::showShutdownWindow(window));
    qDebug() << __func__ << ": Requesting shutdown";
    startThread();
    window->hide();
    
    
    window->unsubscribeFromCoreSignals();
    
    
    m_node.startShutdown();
    
    
    window->setClientModel(nullptr);
    pollShutdownTimer->stop();
    delete clientModel;
    clientModel = nullptr;
    
    Q_EMIT requestedShutdown();
}
void BitcoinApplication::initializeResult(bool success)