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
```


* on the command line or in a network-specific section in the
     * config file.
     */
    const std::set<std::string> GetUnsuitableSectionOnlyArgs() const;
    /**
     * Log warnings for unrecognized section names in the config file.
     */
    const std::list<SectionInfo> GetUnrecognizedSections() const;
    /**
     * Return a vector of strings of the given argument
     *
     * @param strArg Argument to get (e.g. "-foo")
     * @return command-line arguments
     */
    std::vector<std::string> GetArgs(const std::string& strArg) const;
    /**
     * Return true if the given argument has been manually set
     *
     * @param strArg Argument to get (e.g. "-foo")
     * @return true if the argument has been set
     */
    bool IsArgSet(const std::string& strArg) const;
    /**
     * Return true if the argument was originally passed as a negated option,
     * i.e. -nofoo.
     *
     * @param strArg Argument to get (e.g. "-foo")
     * @return true if the argument was passed negated
     */
    bool IsArgNegated(const std::string& strArg) const;