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
```


#endif
#include <chainparams.h>
#include <clientversion.h>
#include <compat.h>
#include <fs.h>
#include <init.h>
#include <interfaces/chain.h>
#include <noui.h>
#include <shutdown.h>
#include <ui_interface.h>
#include <util/strencodings.h>
#include <util/system.h>
#include <util/threadnames.h>
const std::function<std::string(const char*)> G_TRANSLATION_FUN = nullptr;
/*! \mainpage Developer documentation
 *
 * \section intro_sec Introduction
 *
 * This is the developer documentation of the reference client for an experimental new digital currency called Bitcoin,
 * which enables instant payments to anyone, anywhere in the world. Bitcoin uses peer-to-peer technology to operate
 * with no central authority: managing transactions and issuing money are carried out collectively by the network.
 *
 * The software is a community-driven open source project, released under the MIT license.
 *
 * See https:github.com/bitcoin/bitcoin and https:bitcoincore.org/ for further information about the project.
 *
 * \section Navigation
 * Use the buttons <code>Namespaces</code>, <code>Classes</code> or <code>Files</code> at the top of the page to start navigating the code.
 */
static void WaitForShutdown()