Terminal(teletype?) detection implementation is wrong

The current implementation relies on scanning `/proc/{pid}/fd`, which works well in privileged mode because all `fd`'s are accessible.

But the result is incorrect when used in normal user mode. For example:

```shell
❯ pgrep -t tty1
2384
❯ cargo run -q pgrep -t tty1
(*empty here)
```

And affects the following commands

- `pgrep`
- `pidof`
- `pkill` (in future)