authd's password prompt is not shown when stdout is redirected

The password prompt of su is printed to stderr, so it's also shown when stdout is redirected:

```
su > /dev/null
Password:
```

The password prompt of sudo is printed even when both stdout and stderr are redirected:

```
sudo ls >/dev/null 2>&1
[sudo] password for user:
```

For authd users, those password prompts are not shown but instead the password prompt of the authd PAM module is shown:

```
sudo ls
Enter your local password:
>
```

That password prompt is not shown when stdout is redirected:

```
sudo ls > /dev/null
```

In effect, commands like `sudo ls | grep foo` block without printing anything.