This patch introduces two new flags called IPS_PICKUP that forces the
protocol handler to pick up the window of valid TCP packets and
IPS_IN_WINDOW to by pass window checkings.

Moreover, four new attributes to inject the window scale factor and
enable SACK are introduced. These new facilities provide the appropiate
mechanisms to take over TCP connections in failover settings with TCP
tracking enabled.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

-- 
The dawn of the fourth age of Linux firewalling is coming; a time of
great struggle and heroic deeds -- J.Kadlecsik got inspired by J.Morris