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
```


#pragma warning(disable:4717)
#endif
#ifdef _WIN32_IE
#undef _WIN32_IE
#endif
#define _WIN32_IE 0x0501
#define WIN32_LEAN_AND_MEAN 1
#ifndef NOMINMAX
#define NOMINMAX
#endif
#include <codecvt>
#include <io.h> 
#include <shellapi.h>
#include <shlobj.h>
#endif
#ifdef HAVE_MALLOPT_ARENA_MAX
#include <malloc.h>
#endif
#include <thread>
const int64_t nStartupTime = GetTime();
const char * const BITCOIN_CONF_FILENAME = "bitcoin.conf";
ArgsManager gArgs;
/** A map that contains all the currently held directory locks. After
 * successful locking, these will be held here until the global destructor
 * cleans them up and thus automatically unlocks them, or ReleaseDirectoryLocks
 * is called.
 */
static std::map<std::string, std::unique_ptr<fsbridge::FileLock>> dir_locks;
static std::mutex cs_dir_locks;
bool LockDirectory(const fs::path& directory, const std::string lockfile_name, bool probe_only)