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;

Libraries/LibAudio/AClientConnection.cpp

@@ -30,3 +30,19 @@ void AClientConnection::enqueue(const ABuffer& buffer)
         sleep(1);
     }
 }
+
+int AClientConnection::get_main_mix_volume()
+{
+    ASAPI_ClientMessage request;
+    request.type = ASAPI_ClientMessage::Type::GetMainMixVolume;
+    auto response = sync_request(request, ASAPI_ServerMessage::Type::DidGetMainMixVolume);
+    return response.value;
+}
+
+void AClientConnection::set_main_mix_volume(int volume)
+{
+    ASAPI_ClientMessage request;
+    request.type = ASAPI_ClientMessage::Type::SetMainMixVolume;
+    request.value = volume;
+    sync_request(request, ASAPI_ServerMessage::Type::DidSetMainMixVolume);
+}

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);
 };

Libraries/LibAudio/ASAPI.h

@@ -7,11 +7,14 @@ struct ASAPI_ServerMessage {
         PlayingBuffer,
         FinishedPlayingBuffer,
         EnqueueBufferResponse,
+        DidGetMainMixVolume,
+        DidSetMainMixVolume,
     };
 
     Type type { Type::Invalid };
     unsigned extra_size { 0 };
     bool success { true };
+    int value { 0 };
 
     union {
         struct {
@@ -29,10 +32,13 @@ struct ASAPI_ClientMessage {
         Invalid,
         Greeting,
         EnqueueBuffer,
+        GetMainMixVolume,
+        SetMainMixVolume,
     };
 
     Type type { Type::Invalid };
     unsigned extra_size { 0 };
+    int value { 0 };
 
     union {
         struct {