AESinkOSS: Implement Drain()

Tested by @fneufneuf on IRC. Some literature is here: http://www.4front-tech.com/pguide/audio.html

I think using that ioctl as drain is a good idea

```
- Call SNDCTL_DSP_SYNC when you want to wait until all data has been played.
- SNDCTL_DSP_RESET or SNDCTL_DSP_SYNC should be called when the application wants to change sampling parameters (speed, number of channels or number of bits).
```