add set_playback_volume_range and set_capture_volume_range to Selem

[carlgreen]

I wanted to be able to use consistent values for getting and setting volume, which I discovered can be done by setting the range for these. So I implemented snd_mixer_selem_set_playback_volume_range and snd_mixer_selem_set_capture_volume_range for Selem.

[diwic]

Thanks! Let me know if you need a release.