[arch-general] mpd had removed relative volume functionality.

[b4283]

>
> do i understand this correctly:
> you can still change the volume of mpd, but you can only specify absolute values? they left out the relative setting out of the protocol?
>
> Dieter
Yes, this is correct. Absolute values could still be assigned, which 
means that in the future, the relative volume may have to be realized by 
clients instead of mpd.

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b4283