Use different buffer metrics in the PulseAudio plugin

Use different buffer metrics in the PulseAudio plugin

It increases the "pre-buffering level" (i.e. start threshold) to the
full buffer size minus one period. This makes PA work a little bit
more like normal audio devices, and makes a few drop outs go away for
software which uses very small period sizes.

It also increases the initial maximum buffer size, which allows a
small overcommit. That's not really an issue, but cleaner nonetheless
so I smuggled it into this patch.

Also reported in the ALSA BTS:
	https://bugtrack.alsa-project.org/alsa-bug/view.php?id=3578

From: Lennart Poettering <mznyfn@0pointer.de>