Added seed selection to test_hncp_net. For example, with seed 5 it fails on Mac currently (but not 0-4). Sigh.
Added hncp_node_find_neigh_bidir{,2} and using it in hncp_timeout and
hncp_routing. Now the test_hncp_net seems to claim we're back in
business (=things work).
Not publishing neighbor TLVs for not provably reachable neighbors, if not assuming bidirectional reachability.
Added some debug messages to facilitate tracing down bugs.