- some cleanup on music code.

* change the license of the main file because there is no more id-based code here licensed under the GPL.
* moved VM interface definition out of the implementation.
* moved idmus CCMD out of implementation because it is dependent on Doom level definitions.
* moved s_music.cpp into the music folder with the rest of the music code.

src/d_main.cpp

@@ -148,6 +148,7 @@ void DrawFullscreenSubtitle(const char *text);
 void D_Cleanup();
 void FreeSBarInfoScript();
 void I_UpdateWindowTitle();
+void S_ParseMusInfo();
 
 // PRIVATE FUNCTION PROTOTYPES ---------------------------------------------