More changes for dmix plugin:

- added poll_events to main pcm structure
  - added poll_revents callback to pcm->ops structure
  - fixed snd_pcm_wait() - call revents function and enhanced
    error code reporting
DMIX plugin:
  - more complete code (works at least for one instance)
  - still no "mix" code (it's silent)

src/pcm/pcm_copy.c

@@ -163,6 +163,7 @@ static snd_pcm_ops_t snd_pcm_copy_ops = {
 	dump: snd_pcm_copy_dump,
 	nonblock: snd_pcm_plugin_nonblock,
 	async: snd_pcm_plugin_async,
+	poll_revents: snd_pcm_plugin_poll_revents,
 	mmap: snd_pcm_plugin_mmap,
 	munmap: snd_pcm_plugin_munmap,
 };