Implement autosizing TCP socket buffers.

Implement autosizing TCP socket buffers.

Normally the socket buffers are static (either derived from global
defaults or set with setsockopt) and do not adapt to real network
conditions. Two things happen: a) your socket buffers are too small
and you can't reach the full potential of the network between both
hosts; b) your socket buffers are too big and you waste a lot of
kernel memory for data just sitting around.

With automatic TCP send and receive socket buffers we can start with a
small buffer and quickly grow it in parallel with the TCP congestion
window to match real network conditions.

New sysctls are:
  net.inet.tcp.sendbuf_auto=1 (enabled)
  net.inet.tcp.sendbuf_inc=8192 (8K, step size)
  net.inet.tcp.sendbuf_max=16777216 (16M, growth limit)
  net.inet.tcp.recvbuf_auto=1 (enabled)
  net.inet.tcp.recvbuf_inc=16384 (16K, step size)
  net.inet.tcp.recvbuf_max=16777216 (16M, growth limit)

Additionally, add limiters and sanity checks for TCP MSS (maximum segment
size) resource exhaustion attacks.

Obtained-from: FreeBSD