network - Fix race in accept() - try #2

network - Fix race in accept() - try #2

* The last fix wasn't good enough.  Really try to fix it this time.  Use
  a pool token and validate so_head after acquiring it to deal with races,
  interlock against 0-ref races (sockets can be on the so_comp/so_incomp
  queues with 0 references), and use it for the accept predicate.