Use PA_STREAM_EARLY_REQUESTS if available

Use PA_STREAM_EARLY_REQUESTS if available

Use PA_STREAM_EARLY_REQUESTS if PA knows it. PA >= 0.9.12 learned this flag
which will make the buffering more similar to traditional fragment/period-based
scheduling. Instead of delaying buffer fillup until the latest moment possible
this will cause fillups much earlier.

This is useful for compatibility for broken ALSA clients that don't care about
'readability' of the ALSA fd but instead schedule audio via usleep() or a
similar call. (One exampler being mplayer)