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%

Servers/AudioServer/ASMixer.cpp

@@ -71,6 +71,8 @@ void ASMixer::mix()
 
         for (int i = 0; i < mixed_buffer_length; ++i) {
             auto& mixed_sample = mixed_buffer[i];
+
+            mixed_sample.scale(m_main_volume);
             mixed_sample.clip();
 
             i16 out_sample;