AudioServer: Add a "main mix volume" and a simple program to get/set it

Give the mixer a main volume value (percent) that we scale all the
outgoing samples by (before clipping.)

Also add a simple "avol" program for querying and setting the volume:

- "avol" prints the current volume.
- "avol 200" sets the main mix volume to 200%

Libraries/LibAudio/ABuffer.h

@@ -39,6 +39,13 @@ struct ASample {
             right = -1;
     }
 
+    void scale(int percent)
+    {
+        float pct = (float)percent / 100.0;
+        left *= pct;
+        right *= pct;
+    }
+
     ASample& operator+=(const ASample& other)
     {
         left += other.left;