On my Linux Mint 19.1:
```bash
$ umask
0022
```

After the first run of `bitcoin-qt -sysperm=false` the `0700` permissions are expected for both the datadir (`~/.bitcoin`) and the walletdir (`~/.bitcoin/wallets`).

Actually, the directories are created with such access permissions:
```bash
$ stat ~/.bitcoin | grep Uid
Access: (0755/drwxr-xr-x)  Uid: ( 1000/ hebasto)   Gid: ( 1000/ hebasto)
$ stat ~/.bitcoin/wallets | grep Uid
Access: (0755/drwxr-xr-x)  Uid: ( 1000/ hebasto)   Gid: ( 1000/ hebasto)
```

Refs:
- #4286
- #13371
- #13389

NOTE: reading through the initial PR #4286 makes me think it was controversial. Is it a better and simpler way to just get rid of `-sysperm` option? I believe a user who needs this option can easily run `chown` command.
{
    Assert(cmd.find('=') == std::string::npos);
    Assert(cmd.at(0) != '-');
    LOCK(cs_args);
    m_accept_any_command = false; 
    std::map<std::string, Arg>& arg_map = m_available_args[OptionsCategory::COMMANDS];
    auto ret = arg_map.emplace(cmd, Arg{"", help, ArgsManager::COMMAND});
    Assert(ret.second); 
}
void ArgsManager::AddArg(const std::string& name, const std::string& help, unsigned int flags, const OptionsCategory& cat)
{
    Assert((flags & ArgsManager::COMMAND) == 0); 
    
    size_t eq_index = name.find('=');
    if (eq_index == std::string::npos) {
        eq_index = name.size();
    }
    std::string arg_name = name.substr(0, eq_index);
    LOCK(cs_args);
    std::map<std::string, Arg>& arg_map = m_available_args[cat];
    auto ret = arg_map.emplace(arg_name, Arg{name.substr(eq_index, name.size() - eq_index), help, flags});
    assert(ret.second); 
    if (flags & ArgsManager::NETWORK_ONLY) {
        m_network_only_args.emplace(arg_name);
    }
}
void ArgsManager::AddHiddenArgs(const std::vector<std::string>& names)