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
```


{
    HTTPReply *reply = static_cast<HTTPReply*>(ctx);
    if (req == nullptr) {
        /* If req is nullptr, it means an error occurred while connecting: the
         * error code will have been passed to http_error_cb.
         */
        reply->status = 0;
        return;
    }
    reply->status = evhttp_request_get_response_code(req);
    struct evbuffer *buf = evhttp_request_get_input_buffer(req);
    if (buf)
    {
        size_t size = evbuffer_get_length(buf);
        const char *data = (const char*)evbuffer_pullup(buf, size);
        if (data)
            reply->body = std::string(data, size);
        evbuffer_drain(buf, size);
    }
}
#if LIBEVENT_VERSION_NUMBER >= 0x02010300
static void http_error_cb(enum evhttp_request_error err, void *ctx)