mixer: do not unmute to volume 0, instead unmute to max.

It's not very useful for unmute to still result in volume 0.
Plus it makes it easier to enable sound for devices that were
on mute at startup.

git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@37184 b3059339-0415-0410-9bf9-f77b7e298cf2
alsa: Better volume control handling, especially for S/PDIF.

If no mixer channel was specified and PCM does not exist, just
pick the first.
If no volume control is available, at least support mute and full
volume switching if a playback switch exists.
This is also useful for being able to unmute S/PDIF devices.
Especially useful since there seem to be several driver/ALSA bugs
that mean these devices often start as muted after every boot.

git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@37185 b3059339-0415-0410-9bf9-f77b7e298cf2