Far too many clueless users push the volume far too high[1]. Then they
complain that VLC distorts the sound... Really anything above 100% (0dB)
is prone to saturation. In my experience, loss of quality is noticeable
at +6dB and unbearable at +10dB.

For the references:
In VLC 1.1, the UI was restricted to 200% (+6dB) by default,
while the audio output core allowed up to 400% (+12dB).
PulseAudio recommends +11dB.
Microsoft Windows limits to +0dB.

[1] I actually compared VLC 1.1, VLC 2.0 and WMP with an external
decibel metter. They give the same output at 0dB. At 6dB, both VLC
versions give the same decibel readings and slight distortion. So the
complaints are purely caused by confusion and cluelessness from users.

...and simplify accordingly.

This matches the Lua RC behavior.

   - use the overall volume managed by the playlist rather than
     aout_volumeGet() that now only works when a aout is running.
     This is no problem since the skins2 volume slider functionally
     reflects this overall volume managed by the playlist anyway.
   - stick to what include/vlc_aout_intf.h lets us know about volume
     boundaries(as of today, AOUT_VOLUME_DEFAULT and AOUT_VOLUME_MAX)
     and infer volume to be in the [0., AOUT_VOLUME_MAX/AOUT_VOLUME_DEFAULT]
     range as requested by some functions taking a float as parameter.

This is generic and reusable on non-POSIX platforms.

A more proper fix was made in 3567c033.

pb_peak and pf_peak have only two elements.
(regression from dcf3f71b)

Also fixes a mistake in 229c807c
where isCurrent == isSelected

This fixes #7165 and can be backported.

Several of UI & control plugins follow the volume and mute status
through the playlist instead of the aout. This patch works around
that limitation.