alsamixer: add "restore defaults" keystroke

alsa-project/alsa-utils issue #64 was opened from jidanni:

There should be a way to "restore defaults" for all levels for this sound card. And perhaps for each level individually too.

Issue URL : https://github.com/alsa-project/alsa-utils/issues/64 Repository URL: https://github.com/alsa-project/alsa-utils