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.