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
```


{
        LOCK(am.cs_args);
        std::pair<bool,std::string> found_result(false, std::string());
        
        
        
        found_result = GetArgHelper(am.m_override_args, arg, true);
        if (found_result.first) {
            return found_result;
        }
        
        
        
        if (!am.m_network.empty()) {
            found_result = GetArgHelper(am.m_config_args, NetworkArg(am, arg));
            if (found_result.first) {
                return found_result;
            }
        }
        if (UseDefaultSection(am, arg)) {
            found_result = GetArgHelper(am.m_config_args, arg);
            if (found_result.first) {
                return found_result;
            }
        }
        return found_result;
    }