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.
{
    const size_t COOKIE_SIZE = 32;
    unsigned char rand_pwd[COOKIE_SIZE];
    GetRandBytes(rand_pwd);
    std::string cookie = COOKIEAUTH_USER + ":" + HexStr(rand_pwd);
    /** the umask determines what permissions are used to create this file -
     * these are set to 077 in init.cpp unless overridden with -sysperms.
     */
    std::ofstream file;
    fs::path filepath_tmp = GetAuthCookieFile(true);
    file.open(filepath_tmp);
    if (!file.is_open()) {
        LogPrintf("Unable to open cookie authentication file %s for writing\n", fs::PathToString(filepath_tmp));
        return false;
    }
    file << cookie;
    file.close();
    fs::path filepath = GetAuthCookieFile(false);
    if (!RenameOver(filepath_tmp, filepath)) {
        LogPrintf("Unable to rename cookie authentication file %s to %s\n", fs::PathToString(filepath_tmp), fs::PathToString(filepath));
        return false;
    }
    LogPrintf("Generated RPC authentication cookie %s\n", fs::PathToString(filepath));
    if (cookie_out)
        *cookie_out = cookie;
    return true;
}
bool GetAuthCookie(std::string *cookie_out)