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/AClientConnection.h

@@ -12,4 +12,7 @@ public:
 
     virtual void handshake() override;
     void enqueue(const ABuffer&);
+
+    int get_main_mix_volume();
+    void set_main_mix_volume(int);
 };