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(cs_args);
        m_available_args.clear();
    }
    /**
     * Get the help string
     */
    std::string GetHelpMessage() const;
    /**
     * Check whether we know of this arg
     */
    bool IsArgKnown(const std::string& key) const;
};
extern ArgsManager gArgs;
/**
 * @return true if help has been requested via a command-line arg
 */
bool HelpRequested(const ArgsManager& args);
void SetupHelpOptions(ArgsManager& args);
/**
 * Format a string to be used as group of options in help messages
 *
 * @param message Group name (e.g. "RPC server options:")
 * @return the formatted string
 */