audio/out/push: limit fallback sleep time to reasonable limits
audio/out/push: make draining more robust

It was more complicated than it had to be: the audio thread already
determines whether audio has ended, so we can use that. Remove the
separate logic for draining.
etc: add an input config file that reverts recent key binding changes

This is for users who don't like changes. I'm hoping it will make the
process of cleaning up key bindings less bumpy.

It should be mentioned in the release notes of the next release.