* Greatly reduce the complexity of the LWKT messaging and port abstraction.

* Greatly reduce the complexity of the LWKT messaging and port abstraction.
  Significantly reduce the overhead of the subsystem.

* The message abort algorithm has been rewritten.  It now sends a
  separate message to issue the abort instead of trying to requeue
  the original message.  This also means the TAILQ embedded in the
  lwkt_msg structure can be used by unrelated code during processing
  of the message.

* Numerous MSGF_ flags have been removed, and all the LWKT msg/port
  algorithms have been rewritten and simplified.  The message structure
  is now only touched by the current owner in all situations.

* Numerous structural fields have been removed.  In particular, the
  fields used for message abort sequencing have been simplified and
  we do not try to embed a 'command' field in the base LWKT message
  any more.

* Clean up the netmsg abstraction, which is used all over the network stack.
  Instead of trying to overload fields in lwkt_msg we now simply extend
  the base lwkt_msg into struct netmsg.  The function dispatch now takes
  a netmsg and returns void (before we had to return EASYNC), and we no
  longer need weird casts.

  Accept/connect message aborts are now greatly simplified.