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
```


return EXIT_FAILURE;
    }
    std::string method {};
    for(int i = 1; i < argc; ++i) {
        if (!IsSwitchChar(argv[i][0])) {
            if (!method.empty()) {
                tfm::format(std::cerr, "Error: two methods provided (%s and %s). Only one method should be provided.\n", method.c_str(), argv[i]);
                return EXIT_FAILURE;
            }
            method = argv[i];
        }
    }
    if (method.empty()) {
        tfm::format(std::cerr, "No method provided. Run `bitcoin-wallet -help` for valid methods.\n");
        return EXIT_FAILURE;
    }
    
    if (method == "create" && !gArgs.IsArgSet("-wallet")) {
        tfm::format(std::cerr, "Wallet name must be provided when creating a new wallet.\n");
        return EXIT_FAILURE;
    }
    std::string name = gArgs.GetArg("-wallet", "");
    ECCVerifyHandle globalVerifyHandle;
    ECC_Start();
    if (!WalletTool::ExecuteWalletToolFunc(method, name))
        return EXIT_FAILURE;
    ECC_Stop();
    return EXIT_SUCCESS;
}