firing off an os_load, just move the node from its current reservation
to emulab-ops/reloadpending. This moves the operation out of band from
the user's perspective (he gets more immediate response when an
experiment ends, and besides we cannot handle mass reloads anyway, and
so this approach is unusable until Frisbee.

Change the reload_daemon to look for free nodes that need a reload (as
before) *and* nodes in emulab-ops/reloadpending (as put there by nfree).
In this case, the imageid comes from the reloads table instead of the
node-types table. I also updated the reload_daemon to use libdb routines.

Also change testbed/reloading to emulab-ops/reloading. Maybe someday
I'll remove these hardwired strings.