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
```


std::string HelpMessageGroup(const std::string& message);
/**
 * Format a string to be used as option description in help messages
 *
 * @param option Option message (e.g. "-rpcuser=<user>")
 * @param message Option description (e.g. "Username for JSON-RPC connections")
 * @return the formatted string
 */
std::string HelpMessageOpt(const std::string& option, const std::string& message);
/**
 * Return the number of cores available on the current system.
 * @note This does count virtual cores, such as those provided by HyperThreading.
 */
int GetNumCores();
/**
 * .. and a wrapper that just calls func once
 */
template <typename Callable> void TraceThread(const char* name,  Callable func)
{
    util::ThreadRename(name);
    try
    {
        LogPrintf("%s thread start\n", name);
        func();
        LogPrintf("%s thread exit\n", name);
    }
    catch (const boost::thread_interrupted&)
    {
        LogPrintf("%s thread interrupt\n", name);
        throw;