MINOR: tcp: add further tcp info fetchers

MINOR: tcp: add further tcp info fetchers

Adding on to Thierry's work (http://git.haproxy.org/?p=haproxy.git;h=6310bef5)
I have added a few more fetchers for counters based on the tcp_info struct
maintained by the kernel :

  fc_unacked, fc_sacked, fc_retrans, fc_fackets, fc_lost,
  fc_reordering

Two fields were not added because they're version-dependant :
  fc_rcv_rtt, fc_total_retrans

The fields name depend on the operating system. FreeBSD and NetBSD prefix
all the field names with "__" so we have to rely on a few #ifdef for
portability.