starting from netgraph import in 1999.
netstat(8) used pointer to node as node address, oops. That didn't
work, we need the node ID in brackets to successfully address a node.
We can't look into ng_node, due to inability to include netgraph/netgraph.h
in userland code. So let the node make a hint for a userland, storing
the node ID in its private data.
MFC after: 2 weeks