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
```


{
public:
    const int ID_NETWORKINFO = 0;
    const int ID_BLOCKCHAININFO = 1;
    const int ID_WALLETINFO = 2;
    
    UniValue PrepareRequest(const std::string& method, const std::vector<std::string>& args) override
    {
        if (!args.empty()) {
            throw std::runtime_error("-getinfo takes no arguments");
        }
        UniValue result(UniValue::VARR);
        result.push_back(JSONRPCRequestObj("getnetworkinfo", NullUniValue, ID_NETWORKINFO));
        result.push_back(JSONRPCRequestObj("getblockchaininfo", NullUniValue, ID_BLOCKCHAININFO));
        result.push_back(JSONRPCRequestObj("getwalletinfo", NullUniValue, ID_WALLETINFO));
        return result;
    }
    
    UniValue ProcessReply(const UniValue &batch_in) override
    {
        UniValue result(UniValue::VOBJ);
        std::vector<UniValue> batch = JSONRPCProcessBatchReply(batch_in, 3);