normal experiments. I could have rolled this into the existing code,
but that would have been very unpleasant.

Currently, checks every 15 minutes, using the same idle threshold as
idlemail, but sends email only once every 24 hours. There are two
site variables:

  protogeni/idlecheck		- enable/disable PG idle checks.
  protogeni/idlecheck_terminate - enable/disable termination on idle.

Both start as zero when the site variables are added.