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
```


#include <interfaces/node.h>
#include <noui.h>
#include <ui_interface.h>
#include <uint256.h>
#include <util/system.h>
#include <util/threadnames.h>
#include <memory>
#include <QApplication>
#include <QDebug>
#include <QLibraryInfo>
#include <QLocale>
#include <QMessageBox>
#include <QSettings>
#include <QThread>
#include <QTimer>
#include <QTranslator>
#if defined(QT_STATICPLUGIN)
#include <QtPlugin>
#if defined(QT_QPA_PLATFORM_XCB)
Q_IMPORT_PLUGIN(QXcbIntegrationPlugin);
#elif defined(QT_QPA_PLATFORM_WINDOWS)
Q_IMPORT_PLUGIN(QWindowsIntegrationPlugin);
#elif defined(QT_QPA_PLATFORM_COCOA)
Q_IMPORT_PLUGIN(QCocoaIntegrationPlugin);
#endif
#endif
Q_DECLARE_METATYPE(bool*)
Q_DECLARE_METATYPE(CAmount)
Q_DECLARE_METATYPE(uint256)
static QString GetLangTerritory()