alsa: mixer - fix the override_map for the multichannel support

Merged Jaroslav Kysela requested to merge perexg/pulseaudio:mixer8 into master
The override_map variable was changed from bool to mask (unsigned int).
The channel map override settings is handled per number of channels now.

Also added missing override-map.3 .. override-map.8 to the configuration
parser array.

And lastly, use safe values for the min_dB and max_dB fields when
the position mask is unset to avoid breakage for the upper levels.
(Thanks to Wim Taymans for the idea.)

Edited by Jaroslav Kysela

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