on in production!

PER_VNODE_TT handles emulated links by simply adding up the total bandwidth
consumed by a vnode, and the total bandwidth available on each pnode. Of
course, this can lead to false positives (knapsack problem), but that's okay,
because it just means we won't bail up front - we'll have to do the full
annealing pass before we decide it's unmappable.

PER_VNODE_TT now also tries to figure out why a vnode is not mappable - if
there are _no_ pclasses that match one of the restrictions, it will print out
something like:
  *** No possible mapping for nodeA
      Too many links!
  *** No possible mapping for nodeB
      Too much bandwidth on emulated links!

This completes the following two items from assign_todo.txt:

6.   fix PER_VNODE_TT and vclasses
7.   fix PER_VNODE_TT and emulated vlinks