engine: implement basic gamepad gyroscope calibration

Called on game controller becoming active or by user request.
Exposes current calibration state by read-only console variable.

engine/platform/dos/in_dos.c

@@ -91,16 +91,6 @@ void GAME_EXPORT Platform_SetMousePos(int x, int y)
 
 }
 
-int Platform_JoyInit( void )
-{
-	return 0;
-}
-
-void Platform_JoyShutdown( void )
-{
-
-}
-
 void Platform_EnableTextInput( qboolean enable )
 {
 	keystate.chars = enable;