alsa: use (blksz - round) as start threshold.

The threshold making poll() return POLLOUT is one block (according to
alsa library sources). If a poll-based program writes blksz - 1
samples, playback never starts but poll won't return POLLOUT either.
In turn the program deadlocks. The problem could be observed in the
sio_alsa_xrun() routine.

libsndio/sio_alsa.c

@@ -875,7 +875,7 @@ sio_alsa_setpar(struct sio_hdl *sh, struct sio_par *par)
 			return 0;
 		}
 		err = snd_pcm_sw_params_set_start_threshold(hdl->opcm,
-		    oswp, hdl->par.bufsz);
+		    oswp, hdl->par.bufsz - hdl->par.round);
 		if (err < 0) {
 			DALSA("couldn't set play start threshold", err);
 			hdl->sio.eof = 1;