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
```


error = "Failed to include configuration file " + to_include;
                    return false;
                }
            }
            
            includeconf = GetArgs("-includeconf");
            {
                std::vector<std::string> includeconf_net(GetArgs(std::string("-") + chain_id + ".includeconf"));
                includeconf.insert(includeconf.end(), includeconf_net.begin(), includeconf_net.end());
                std::string chain_id_final = GetChainName();
                if (chain_id_final != chain_id) {
                    
                    includeconf_net = GetArgs(std::string("-") + chain_id_final + ".includeconf");
                    includeconf.insert(includeconf.end(), includeconf_net.begin(), includeconf_net.end());
                }
            }
            for (const std::string& to_include : includeconf) {
                tfm::format(std::cerr, "warning: -includeconf cannot be used from included files; ignoring -includeconf=%s\n", to_include.c_str());
            }
        }
    }
    
    ClearDatadirCache();
    if (!CheckDataDirOption()) {
        error = strprintf("specified data directory \"%s\" does not exist.", gArgs.GetArg("-datadir", "").c_str());
        return false;
    }
    return true;
}
std::string ArgsManager::GetChainName() const