Add subnode support. If a pnode's type ase the subnode flag set, tell assign what it's a subnode of. Also, if a subnode's parent is not going to be included in the ptop file, don't include the subnode either.
Add subnode support. If a pnode's type ase the subnode flag set, tell
assign what it's a subnode of. Also, if a subnode's parent is not
going to be included in the ptop file, don't include the subnode
either.

Also, re-structure the part in which we generate the 'node' line to
make it cleaner, since it's gotten more complex than that code
originally intended.

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