Define sysctl (tcp_cookie_size) to turn on and off the cookie option
default globally, instead of a compiled configuration option.

Define per socket option (TCP_COOKIE_TRANSACTIONS) for setting constant
data values, retrieving variable cookie values, and other facilities.

This is a straightforward re-implementation of an earlier (year-old)
patch that no longer applies cleanly, with permission of the original
author (Adam Langley).  The patch was previously reviewed:

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

These functions will also be used in subsequent patches that implement
additional features.
---
   include/linux/tcp.h        |   31 ++++++++++++++++++++++++++++++-
   include/net/tcp.h          |    1 +
   net/ipv4/sysctl_net_ipv4.c |    8 ++++++++
   net/ipv4/tcp_output.c      |    8 ++++++++
   4 files changed, 47 insertions(+), 1 deletions(-)