command: make volume/mute unavailable if audio is not initialized

This does nothing good. This reverts a change made over a year ago - I
don't remember why this was originally done this way.

The main problem is that even if the volume option is set (something
like "--volume=75"), the volume property will always return "100" until
audio is initialized. If audio is uninitialized again, the volume
property will remain frozen at its last value.
command: don't display non-sense if volume is unavailable

Showed "Volume: (unavailable)%". That was dumb.

The message string is now a bit convoluted; mostly because the property
expand syntax can't do "if-else", just "if".

CC: @mpv-player/stable