alsa-mixer: Implement support for setting element specific upper limits for volume.

This feature is mainly useful in embedded systems that have built-in speakers.
In such situations the full audio path is known beforehand, so it's possible to
know what is the maximum sensible volume, and any higher volume can be
disabled.

The volume limit is set in path configuration files in the [Element] section,
using option "volume-limit". The value is the desired maximum volume step of
the volume element.

src/modules/alsa/alsa-mixer.h

@@ -142,6 +142,7 @@ struct pa_alsa_element {
 
     pa_bool_t has_dB:1;
     long min_volume, max_volume;
+    long volume_limit; /* -1 for no configured limit */
     double min_dB, max_dB;
 
     pa_channel_position_mask_t masks[SND_MIXER_SCHN_LAST][2];