command: always allow setting volume/mute properties

This seems generally easier when using libmpv (and was already requested
and implemented before: see commit 327a779a; it was reverted some time
later).

With the weird internal logic we have to deal with, in particular the
--softvol=no case (using system volume), and using the audio API's mixer
(--softvol=auto on some systems), we still can't avoid all glitches and
corner cases that complicate this issue so much. The API user is either
recommended to use --softvol=yes or auto, or to watch the new
mixer-active property, and assume the volume/mute properties have
significant values if the mixer is active.

Remaining glitches:
- changing the volume/mute properties has no effect if no internal mixer
  is used (--softvol=no) and the mixer is not active; the actual mixer
  controls do not change, only the property values
- --volume/--mute do not have an effect on the volume/mute properties
  before mixer initialization (the options strictly are only applied
  during mixer init)
- volume-max is 100 while the mixer is not active

audio/mixer.c

@@ -399,4 +399,5 @@ void mixer_uninit_audio(struct mixer *mixer)
     }
     mixer->ao = NULL;
     mixer->af = NULL;
+    mixer->softvol = false;
 }