This is a little bit of future-proofing, but also gives us a little more
flexibility in general; now we can add in the cvars to open a specific
device, etc, that the OpenAL codepath does.
In SDL2, the initialized subsystems are referenced counted, so it's safe to
initialize them twice, and it makes the SDL_QuitSubSystem during our shutdown
correctly decrement the count. Before (as a probably-harmless bug), it would
not increment the refcount if the subsystem was already initialized, causing
problems when it decremented it later.
