audio: change default preference order of AO drivers

Now PulseAudio is preferred over ALSA, which in turn is preferred over
OSS. This should give best results on all systems. On systems with
PulseAudio, we will always use it natively, rather than through the
suboptimal ALSA emulation (which the default ALSA output is normally
redirected to when PulseAudio is active; ALSA hardware devices will
not be, but to use those the user must set AO explicitly in any case,
so changing the defaults makes no difference). The fallback from
ao_pulse to ao_alsa causes no noticeable delay on systems without
PulseAudio. On systems with ALSA, we won't attempt to use OSS anymore.

Also, move OpenAL above SDL. OpenAL should generally work better than
SDL.

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