* New option -h to turn on shared nodes; set by the mapper wrapper
  when it sees that an experiment wants to use shared nodes.

* When using shared nodes, look for physical nodes that are reserved,
  but have their sharing mode set. The node must also be up.

* Shared hosts nodes get the feature pcshared:1.0 in the ptop
  file. libvtop adds the corresponding desire.

* On the shared hosts, do not spit all the OSIDs, but just the one it
  is actually running. This allows the user to indicate what kind of
  shared node they want via the osid. libvtop adds that as a desire.
  Crude but effective.

* Use the interface_state table to get the remaining bandwidth on the
  interfaces for shared hosts, so that assign does not oversubscribe
  links.

* Some initial code to look at the load averages on shared nodes, to
  guide the packing. Not actually operation yet.

  At the moment packing is still controlled by the collocate factor
  from the experiment (NS file). Eventually we want to use the load
  averages above, or other feature/desire mechanisms to control
  packing.