LWKT message ports contain a number of function pointers which abstract

LWKT message ports contain a number of function pointers which abstract
their backend operation.

* Add a new function, mp_getport(), which takes over the functionality
  of lwkt_getport().

* Formalize the default backend and rename it the 'thread' port backend, used
  when a message port will only be drained by a single thread.  This backend
  is able to use critical sections and IPI messages to handle races.

* Fix a small timing window in the thread port backend where replying a
  synchronous message request from a different cpu may fail to wake up
  the originator who is waiting for the message completion.

* Abstract-out the message port initialization code and clean up related
  code pollution.

* Add a new backend called the 'spin' port backend.  This backend can be
  used if a message port might be drained by several different threads.
  For example, this would allow us to use a message port as part of a
  file pointer / file descriptor construct.

* Add a boot-time tunable, lwkt.use_spin_port (defaults to off) which
  forces spin ports to be used instead of thread ports for the per-thread
  message port.  This is used only for debugging.