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
```


else
        {
            strUsage += "\nUsage:  bitcoind [options]                     Start " PACKAGE_NAME " Daemon\n";
            strUsage += "\n" + gArgs.GetHelpMessage();
        }
        tfm::format(std::cout, "%s", strUsage.c_str());
        return true;
    }
    try
    {
        if (!CheckDataDirOption()) {
            return InitError(strprintf("Specified data directory \"%s\" does not exist.\n", gArgs.GetArg("-datadir", "")));
        }
        if (!gArgs.ReadConfigFiles(error, true)) {
            return InitError(strprintf("Error reading configuration file: %s\n", error));
        }
        
        try {
            SelectParams(gArgs.GetChainName());
        } catch (const std::exception& e) {
            return InitError(strprintf("%s\n", e.what()));
        }
        
        for (int i = 1; i < argc; i++) {
            if (!IsSwitchChar(argv[i][0])) {
                return InitError(strprintf("Command line contains unexpected token '%s', see bitcoind -h for a list of options.\n", argv[i]));
            }
        }
        
        gArgs.SoftSetBoolArg("-server", true);