netfilter: xt_CT: add alias flag

This patch adds the alias flag to support full NOTRACK target
aliasing.

Based on initial patch from Jozsef Kadlecsik.

Acked-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hi>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>