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:
    UniValue PrepareRequest(const std::string& method, const std::vector<std::string>& args) override
    {
        UniValue params;
        if(gArgs.GetBoolArg("-named", DEFAULT_NAMED)) {
            params = RPCConvertNamedValues(method, args);
        } else {
            params = RPCConvertValues(method, args);
        }
        return JSONRPCRequestObj(method, params, 1);
    }
    UniValue ProcessReply(const UniValue &reply) override
    {
        return reply.get_obj();
    }
};
static UniValue CallRPC(BaseRequestHandler *rh, const std::string& strMethod, const std::vector<std::string>& args)