af_volume: fix replaygain

This was accidentally broken in commit b72ba3f7. I somehow made the
wild assumption that replaygain adjusted the volume relative to 0%
instead of 100%.

The detach suboption was similarly broken.
build: check if replaygain side data is available
demux: add replaygain_data field to demuxer_t
af: add replaygain_data field to af_stream and af_instance

Closes #664
af_volume: use replaygain side data