kernel - Add feature to allow sendbuf_auto to decrease the buffer size

* sysctl net.inet.tcp.sendbuf_auto (defaults to 1) is now able to
  decrease the tcp buffer size as well as increase it.

* Inflight bwnd data is used to determine how much to decrease the
  buffer.  Inflight is enabled by default.  If you disable it
  with (net.inet.tcp.inflight_enable=0), sendbuf_auto will not
  be able to adjust buffer sizes down.

* Set net.inet.tcp.sendbuf_min (default 32768) to set the floor for
  any downward adjustment.

* Set net.inet.tcp.sendbuf_auto=2 to disable the decrease feature.