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
```


#if defined(HAVE_CONFIG_H)
#include <config/bitcoin-config.h>
#endif
#include <chainparamsbase.h>
#include <clientversion.h>
#include <fs.h>
#include <rpc/client.h>
#include <rpc/protocol.h>
#include <rpc/request.h>
#include <util/system.h>
#include <util/strencodings.h>
#include <memory>
#include <stdio.h>
#include <tuple>
#include <event2/buffer.h>
#include <event2/keyvalq_struct.h>
#include <support/events.h>
#include <univalue.h>
const std::function<std::string(const char*)> G_TRANSLATION_FUN = nullptr;
static const char DEFAULT_RPCCONNECT[] = "127.0.0.1";
static const int DEFAULT_HTTP_CLIENT_TIMEOUT=900;
static const bool DEFAULT_NAMED=false;
static const int CONTINUE_EXECUTION=-1;
static void SetupCliArgs()