ALSA plugin fixes
The attached patch makes various enhancements to libao's ALSA plugin:
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it isn't necessary to use a separate snd_pcm_access_mask_t variable to set the access mask;
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add a sample rate check (similar to the OSS plugin);
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return the correct error code when calling snd_pcm_prepare() after an underrun;
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set the start_threshold to a positive value – using zero can result in an immediate underrun when the first write is very small;
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various comments were wrong or misleading.