[CFR] Forward RTO recovery algorithm (rfc5682) patch

This patch is *VERY* experimental patch to implement rfc5862 which is on
the IETF standard tracks so it could be completely wrong or has a lot of
bugs on it or wrong approaches because I'm a really newbie on TCP stack.

This patch includes two features to support `Basic FRTO algorithm' and
`SACK-Enhanced FRTO algorithm' but not including a feature to support
things mentioned at `Appendix A'.

I'm looking for reviewers teaching me where it's wrong or what the
approach was bad on line by line.  However any comments are welcome.

The patch is available at:

        http://people.freebsd.org/~weongyo/patch_tcpfrto_20110205.diff

regards,
Weongyo Jeong