ospfd: Reset neighbour inactivity timer for any packet arrival

* The hello protocol monitors connectivity in 2 different ways:

  a) local -> remote
  b) remote -> local

  Connectivity is required in both directions (2-way) for adjacencies to
  form.

  The first requires a round-trip to detect, and is done by advertising
  which other hosts a router knows about in its hello messages.  This allows
  a host to detect which other routers are and are not receiving its
  message.  If a remote neighbour delists the local router, then the local
  router raises a "1-Way Received" event.

  The latter is straight-forward, and is detected by setting a timer for the
  neighbour. If another Hello packet is not received within this time then
  the neighbour is dead, and a separate "Inactive" event is raised.

  These are 2 different and relatively independent measures.

  Knowing that we can optimise the 2nd, remote->local measure and reset
  the timer when /any/ packet arrives from that neighbour. For any packet
  is as good as a Hello packet. This can help in marginal situations, where
  the number of protocol messages that must be sent sometimes can exceed
  the capacity of the network to transmit the messages within the configured
  dead-time. I.e. an OSPF network with lots of LSAs, slow links and/or
  slow hosts (e.g. O(10k) LSAs, O(100kbit) links, embedded CPUs, and O(10s)
  dead-times).

  This optimisation allows an OSPF network to run closer to this margin,
  and/or allows networks to perhaps better cope with rare periods of
  exceptional load, where otherwise they would not.

  It's fully compatible with plain OSPF implementations and doesn't
  prejudice dead-neighbour detection.

* ospf_nsm.h: Rename HelloReceived event to PacketReceived.
* ospf_nsm.c: (nsm_hello_received) -> nsm_packet_received
* ospf_packet.c: Schedule PacketReceived whenever a valid message is
  received.

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