Introduce a `u32' match which allows to extract quantities of up to 4
bytes from a packet and test whether the result is a certain value.

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