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.
{
    CSHA256 hasher;
    hasher.Write(dest.data(), dest.size());
    unsigned char hash[CSHA256::OUTPUT_SIZE];
    hasher.Finalize(hash);
    CNetAddr addr;
    const std::string addr_str = EncodeBase32(hash, false) + ".b32.i2p";
    if (!addr.SetSpecial(addr_str)) {
        throw std::runtime_error(strprintf("Cannot parse I2P address: \"%s\"", addr_str));
    }
    return addr;
}
/**
 * Derive the .b32.i2p address of an I2P destination (I2P-style Base64).
 * @param[in] dest I2P destination.
 * @return the address that corresponds to `dest`
 * @throw std::runtime_error if conversion fails
 */
static CNetAddr DestB64ToAddr(const std::string& dest)