Various little fixes

Hi,

I forked pnmixer just to add support for USB soundcards that use the softvol alsa plugin, that is in the first commit. But then I grew interested a little bit more and found myself fixing every little thing that needed it. I hope you appreciate :)

Say, do you use the hotkey feature of pnmixer ? It doesn't work for me. The keys are grabbed properly, but the callback 'key_filter()' is never invoked. I just wondered if it worked OK for you or if it was currently broken.

Cheers,
Arnaud