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
```


{
#ifdef WIN32
    return c == '-' || c == '/';
#else
    return c == '-';
#endif
}
enum class OptionsCategory {
    OPTIONS,
    CONNECTION,
    WALLET,
    WALLET_DEBUG_TEST,
    ZMQ,
    DEBUG_TEST,
    CHAINPARAMS,
    NODE_RELAY,
    BLOCK_CREATION,
    RPC,
    GUI,
    COMMANDS,
    REGISTER_COMMANDS,
    HIDDEN 
};
struct SectionInfo
{
    std::string m_name;
    std::string m_file;
    int m_line;
};
class ArgsManager