* Add a "showclass" param to shownodes so that the
   node listing will include only nodes of the indicated class.
   Prepend "no-" to exclude nodes of a particular class.  So:
        showclass=pc,mote
   would show just pcs and motes.

 * Now the real reason I added the above: change the hwdown experiment
   node list to include "showclass=no-pcplabphys" by default so I don't have
   to flip through 600+ dead plab nodes.  Change the URL to "showclass=all"
   if you want to see plab nodes in hwdown.