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
```


/**
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     *
     * @param strArg Argument to get (e.g. "-foo")
     * @param strDefault (e.g. "1")
     * @return command-line argument or default value
     */
    std::string GetArg(const std::string& strArg, const std::string& strDefault) const;
    /**
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     *
     * @param strArg Argument to get (e.g. "-foo")
     * @param nDefault (e.g. 1)
     * @return command-line argument (0 if invalid number) or default value
     */
    int64_t GetArg(const std::string& strArg, int64_t nDefault) const;
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     * @param strArg Argument to get (e.g. "-foo")
     * @param fDefault (true or false)
     * @return command-line argument or default value
     */
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     * @param strValue Value (e.g. "1")
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