Currently the plugout_alsa "pauses" playback simply by ceasing to
call `snd_pcm_writei()`; this causes a buffer underrun error on
resuming playback, and omission of one buffer's worth of samples
from playback (currently 2048 samples).

To mitigate this, check for an underrun retval (`-EPIPE`) after calling
`snd_pcm_writei()` and if applicable, recover by calling
`snd_pcm_prepare()` and	repeating the `snd_pcm_writei()` call to avoid
dropping samples.

Also implement an `alsa_pause()` function to pause and resume playback
without a buffer underrun using `snd_pcm_pause()`. The call will be
ineffective if unsupported by underlying hardware, in which case the
buffer underrun handling above will come into play.

Don't try to pause the ALSA sound device if the device does not
support the pause operation.

Also report errors on pause/resume.