Add very simple minded SpanningTree() function, which creates a loopfree path from a set of trunk links.
Add very simple minded SpanningTree() function, which creates a
loopfree path from a set of trunk links.

Silly place for this, but I need it in libvtop.

nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome