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 {
        strRPCUserColonPass = gArgs.GetArg("-rpcuser", "") + ":" + gArgs.GetArg("-rpcpassword", "");
    }
    struct evkeyvalq* output_headers = evhttp_request_get_output_headers(req.get());
    assert(output_headers);
    evhttp_add_header(output_headers, "Host", host.c_str());
    evhttp_add_header(output_headers, "Connection", "close");
    evhttp_add_header(output_headers, "Authorization", (std::string("Basic ") + EncodeBase64(strRPCUserColonPass)).c_str());
    
    std::string strRequest = rh->PrepareRequest(strMethod, args).write() + "\n";
    struct evbuffer* output_buffer = evhttp_request_get_output_buffer(req.get());
    assert(output_buffer);
    evbuffer_add(output_buffer, strRequest.data(), strRequest.size());
    
    std::string endpoint = "/";
    if (!gArgs.GetArgs("-rpcwallet").empty()) {
        std::string walletName = gArgs.GetArg("-rpcwallet", "");
        char *encodedURI = evhttp_uriencode(walletName.c_str(), walletName.size(), false);
        if (encodedURI) {
            endpoint = "/wallet/"+ std::string(encodedURI);
            free(encodedURI);
        }
        else {
            throw CConnectionFailed("uri-encode failed");
        }
    }
    int r = evhttp_make_request(evcon.get(), req.get(), EVHTTP_REQ_POST, endpoint.c_str());
    req.release(); 
    if (r != 0) {
        throw CConnectionFailed("send http request failed");