switch to pacmd for muting sound on cluster machines

[geofft]

I wrote in #292:

There's a related issue that this doesn't do anything on the Dell 760s because they have an entirely different "Mono" slider all the way to the right of alsamixer that's off by default, but making unmuting sane across the different hardware types is a different ticket.

If we use PulseAudio's pacmd set-sink-mute 0 1 instead of /etc/init.d/alsa-utils stop, we mute sound the way that checking the "mute" checkbox on the panel volume control widget would do that, as opposed to muting every ALSA slider so that you have to go unmute them all.

I'd make this change right now, but I'm in the middle of an invasive rewrite of debathena-reactivate, and I also worry that you'd need to be clever about restoring ALSA levels, since alsa-utils stop has the property of saving the previous levels — so if you run it twice in a row, you save the muted levels. We'll want to have ALSA's saved levels be full / default volume before switching to pacmd.