Terminal not restored properly when parallel processing

```bash
$ printf "MUSICIANS/G/Galway_Martin/Arkanoid.sid\nMUSICIANS/D/Daglish_Ben/Kettle.sid\n" | xargs -n1 -P2 sidplayfp -os -t1 -w
$ $ $ $ $ stty # TERMINAL NOT RESTORED. PRESSING ENTER MAKES NO NEWLINE
lflags: -icanon -iexten -echo echoe echoke echoctl pendin
$ stty sane
lflags: echoe echoke echoctl pendin
$

```

https://github.com/libsidplayfp/sidplayfp/blob/master/src/keyboard.cpp#L287
