Add a missing <string.h>
Fix node::port_number to actually work

This was comparing for the wrong string.  I guess I should test things
I write... :).
Add ocn-node_list and ocn-neighbors

These two programs print some information that's hopefully useful:

* ocn-node_list prints all the nodes in a network
* ocn-neighbors prints all the neighbors of every node

ocn-neighbors should be the canonical way to define a OCN network as a
text file, but I don't actually have a way to read it back in yet...