#54 non-blocking ioctl fixes

[Stas Sergeev]

This patch is necessary to get a dosemu-1.1.4+
to work with aureal drivers.
It changes the following:
- SNDCTL_DSP_POST must always be non-blocking
- SNDCTL_DSP_POST must not interrupt the output
- SNDCTL_DSP_SYNC must always be blocking
- drain_playback() must increase a fill_index
after setting the "in_use" for the current block.

[Stas Sergeev]

Patch to make aureal driver dosemu-friendly