Audio: misc bug fixes

* Add more const for Buffer::Feed and AL::ListenerOrientation
* Check that the system is initialized in StartMusic
* Make Shutdown shutdown everything
* StreamingSound restarts the source if it had consumed all the buffers
Audio: better mimic the looping sound interface of the previous sound system

If several loops are added in a frame, the last one added gets played.
Also replaced a Log::Warn with audioLogs.Warn and fixed a bug at exit
in the capture.