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
```


throw std::runtime_error("-stdinrpcpass specified but failed to read from standard input");
            }
            gArgs.ForceSetArg("-rpcpassword", rpcPass);
        }
        std::vector<std::string> args = std::vector<std::string>(&argv[1], &argv[argc]);
        if (gArgs.GetBoolArg("-stdin", false)) {
            
            std::string line;
            while (std::getline(std::cin, line)) {
                args.push_back(line);
            }
        }
        std::unique_ptr<BaseRequestHandler> rh;
        std::string method;
        if (gArgs.GetBoolArg("-getinfo", false)) {
            rh.reset(new GetinfoRequestHandler());
            method = "";
        } else {
            rh.reset(new DefaultRequestHandler());
            if (args.size() < 1) {
                throw std::runtime_error("too few parameters (need at least command)");
            }
            method = args[0];
            args.erase(args.begin()); 
        }
        
        const bool fWait = gArgs.GetBoolArg("-rpcwait", false);
        do {
            try {
                const UniValue reply = CallRPC(rh.get(), method, args);