TCP - Fix problems with dynamic receiver and sender buffer scaling

TCP - Fix problems with dynamic receiver and sender buffer scaling

* The receive buffer dynamic scaling ran afoul of our
  avoid_pure_win_update feature.  Correct the calculation so this
  feature properly works when the maximum receive window is smaller
  the maximum sockbuf size.

* Both the send and receive buffer dynamic scaling were limited
  to a maximum TCP window based on the initial window scaling negotiation,
  which in turn is based on the initial size of the receive or send
  sockbuf.

  Negotiate a minimum window scaling of 6 (x64).  This currently limits
  automatic sockbuf expansion to 4MB.  Further refinement is possible
  as long as (1 << scaling) does not exceed 1/4 of the initial sockbuf
  size.

* Reduce the maximum size of sockbuf can be dynamically scaled to from
  16MB to 2MB.  If someone wants more they can sysctl it up higher.

* Disable further scaling once the sockbuf size has exceeded the
  originally negotiated window scale parameters.  There's no gain in
  doing it past that.