Calculate and format TCP_COOKIE SYNACK option.

Process optional SYNACK data.

This is a significantly revised implementation of an earlier (year-old)
patch that no longer applies cleanly, with permission of the original
author (Adam Langley):

    http://thread.gmane.org/gmane.linux.network/102586

Requires:
   TCPCT part 1a: add request_values parameter for sending SYNACK
   TCPCT part 1b: generate Responder Cookie
   TCPCT part 1c: sysctl_tcp_cookie_size, socket option TCP_COOKIE_TRANSACTIONS
   TCPCT part 1d: define TCP cookie option, extend existing struct's
   TCPCT part 1e: implement socket option TCP_COOKIE_TRANSACTIONS
   TCPCT part 1f: Initiator Cookie => Responder

Signed-off-by: William.Allen.Simpson@gmail.com
---
  include/linux/tcp.h      |   11 ++++++
  include/net/tcp.h        |    1 +
  net/ipv4/syncookies.c    |    5 ++-
  net/ipv4/tcp_input.c     |   84 ++++++++++++++++++++++++++++++++++++++++------
  net/ipv4/tcp_ipv4.c      |   46 +++++++++++++++++++++++--
  net/ipv4/tcp_minisocks.c |   14 ++++---
  net/ipv4/tcp_output.c    |   75 ++++++++++++++++++++++++++++++++++++++--
  net/ipv6/syncookies.c    |    5 ++-
  net/ipv6/tcp_ipv6.c      |   51 +++++++++++++++++++++++++++-
  9 files changed, 261 insertions(+), 31 deletions(-)