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
```


{
    try
    {
        qDebug() << __func__ << ": Running initialization in thread";
        util::ThreadRename("qt-init");
        bool rv = m_node.appInitMain();
        Q_EMIT initializeResult(rv);
    } catch (const std::exception& e) {
        handleRunawayException(&e);
    } catch (...) {
        handleRunawayException(nullptr);
    }
}
void BitcoinCore::shutdown()
{
    try
    {
        qDebug() << __func__ << ": Running Shutdown in thread";
        m_node.appShutdown();
        qDebug() << __func__ << ": Shutdown finished";
        Q_EMIT shutdownResult();
    } catch (const std::exception& e) {
        handleRunawayException(&e);
    } catch (...) {
        handleRunawayException(nullptr);
    }
}
BitcoinApplication::BitcoinApplication(interfaces::Node& node, int &argc, char **argv):