MFC 1.70:

MFC 1.70:
  Fix several races between socket closure and node/hook
  destruction:
    - Backout 1.62, since it doesn't fix all possible
    problems.
    - Upon node creation, put an additional reference on node.
    - Add a mutex and refcounter to struct ngsock. Netgraph node,
      control socket and data socket all count as references.
    - Introduce ng_socket_free_priv() which removes one reference
      from ngsock, and frees it when all references has gone.
    - No direct pointers between pcbs and node, all pointing
      is done via struct ngsock and protected with mutex.