Split the enable-lfe-remixing setting into two

remixing-produce-lfe controls upmixing, and remixing-consume-lfe controls downmixing. The motivation is that a user might want to synthesize LFE while playing stereo audio on his/her 5.1 speakers, but at the same time follow the industry recommendation to omit the LFE channel when producting a stereo downmix (e.g. for headphones) from 5.1 content. Or the other way round.

Fixes: #753 (closed).