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.
unsigned int flags, std::string& error)
{
    
    if (key.negated) {
        if (flags & ArgsManager::DISALLOW_NEGATION) {
            error = strprintf("Negating of -%s is meaningless and therefore forbidden", key.name);
            return std::nullopt;
        }
        
        if (value && !InterpretBool(*value)) {
            LogPrintf("Warning: parsed potentially confusing double-negative -%s=%s\n", key.name, *value);
            return true;
        }
        return false;
    }
    if (!value && (flags & ArgsManager::DISALLOW_ELISION)) {
        error = strprintf("Can not set -%s with no value. Please specify value with -%s=value.", key.name, key.name);
        return std::nullopt;
    }
    return value ? *value : "";
}
ArgsManager::ArgsManager() = default;
ArgsManager::~ArgsManager() = default;