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%

Userland/avol.cpp

@@ -0,0 +1,19 @@
+#include <LibAudio/ABuffer.h>
+#include <LibAudio/AClientConnection.h>
+#include <stdio.h>
+
+int main(int argc, char** argv)
+{
+    CEventLoop loop;
+    AClientConnection a_conn;
+    a_conn.handshake();
+
+    if (argc > 1) {
+        int new_volume = atoi(argv[1]);
+        a_conn.set_main_mix_volume(new_volume);
+    }
+
+    int volume = a_conn.get_main_mix_volume();
+    printf("Volume: %d\n", volume);
+    return 0;
+}