TCP: Add support for Linux TCP_FASTOPEN (aka TFO)

Recent Linux Kernels implements a new TCP feature
called TCP_FASTOPEN. This feature reduces the TCP
handshake roundtrip improving performance for new
connections. For more details refer to the following
articles:

 - http://lwn.net/Articles/508865/
 - http://kernelnewbies.org/Linux_3.7#head-cd32b65674184083465d349ad6d772c828fbbd8b

Requirements:

 - the client side must be running Kernel >= 3.6.
 - the server side must be running Kernel >= 3.7.
 - Besides the Kernel version, the client and the server
   must request to use TCP_FASTOPEN/MSG_FASTOPEN in their
   respective calls.
 - the tcp_fastopen flag must be enabled:

      # echo 1 > /proc/sys/net/ipv4/tcp_fastopen

The good thing of this feature is that no matters if
the client implements TFO or not, the server will work
anyways.

This patch enable TCP_FASTOPEN by default in Liana plugin
and also export a new function mk_socket_set_tcp_fastopen()
for generic purposes.

Signed-off-by: Eduardo Silva <edsiper@gmail.com>