Re-implement smooth hardware fade-up to previous value after audio graph has been set up.

[nettings]

We should boot with hardware mixer set to -oo dB, and only fade up gently after everything is stable (don't just switch on, some stream might already be playing, and we don't want to frighten anyone).

[nettings]

This issue has become less pressing now that we disconnect all processes before killing them, which prevents buffer motorboating. But for startup it would still be kind of nice. The big question is where, since audio hardware is quite variable. Parsing effect_0 settings (the matrixmixer) from /local/data/mod-host.state and ramping those would deal with all signals, not just the hardware outs - might be useful for mn_zita-j2n senders, too.