Add UserBufferManagementInit()

Used to reset any state required in UserBufferManagement(). Weak default
implementation provided, which does nothing.

Also tidied up weak default implementation of UserBufferManagement(),
and added a header file to define both function protypes.

module_usb_audio/audio_io/userbuffermanagement.h

@@ -0,0 +1,13 @@
+#ifndef _USERBUFFERMANAGEMENT_H_
+#define _USERBUFFERMANAGEMENT_H_
+
+#include "xua_audio.h"
+#include <xccompat.h>
+
+void UserBufferManagementInit(CLIENT_INTERFACE(audManage_if, i_audMan));
+
+void UserBufferManagement(unsigned sampsFromUsbToAudio[],
+                          unsigned sampsFromAudioToUsb[],
+                          CLIENT_INTERFACE(audManage_if, i_audMan));
+
+#endif // _USERBUFFERMANAGEMENT_H_