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
```


{
    Q_UNUSED(context);
    if (type == QtDebugMsg) {
        LogPrint(BCLog::QT, "GUI: %s\n", msg.toStdString());
    } else {
        LogPrintf("GUI: %s\n", msg.toStdString());
    }
}
BitcoinCore::BitcoinCore(interfaces::Node& node) :
    QObject(), m_node(node)
{
}
void BitcoinCore::handleRunawayException(const std::exception *e)
{
    PrintExceptionContinue(e, "Runaway exception");
    Q_EMIT runawayException(QString::fromStdString(m_node.getWarnings("gui")));
}
void BitcoinCore::initialize()