Rename tcp_sndbuf_autoscale() to tcp_autosndbuf(). This matches
complementary tcp_autorcvbuf() that was factored out as a separate
function a few months before tcp_sndbuf_autoscale() was. Move the large
comment to the header of the function. Use unsigned types for all sizing
sysctls and variables and use bool for the switching sysctl. Add a
comment above net.inet.tcp.sendbuf_auto_lowat (default to off) providing a
reference to its origins. Add DTrace probe tcp::send-autoresize,
complementary to the existing tcp::receive-autoresize.
Rack has its own function, just enable the DTrace probe there.