support TCP Fast Open option

Fast Open" is a optimization to the process of stablishing a TCP connection that
allows the elimination of one round time trip (RTT) from certain kinds of TCP
conversations. Fast Open could result in speed improvements of between 4% and
41% in the page load times on popular web sites.

Available on linux >= 3.7.1.

More info:

- https://lwn.net/Articles/508865/
- http://research.google.com/pubs/pub37517.html