[vdr] remember volume

[Joerg Riechardt <J.Riechardt@gmx.de>]

Hello Klaus,
I would like to have an adition to the audio control.
Problem: I use audio at around 40%. After restarting (driver + vdr) its
again at 100%.
Pain in the ears. It would be nice, if VDR remembers the last audio
volume, like it remembers the last channel.
Jörg