netfilter: xt_CT: fix assignation of the generic protocol tracker

`iptables -p all' uses 0 to match all protocols, while the conntrack
subsystem uses 255. We still need `-p all' to attach the custom
timeout policies for the generic protocol tracker.

Moreover, we may use `iptables -p sctp' while the SCTP tracker is
not loaded. In that case, we have to default on the generic protocol
tracker.

Another possibility is `iptables -p ip' that should be supported
as well. This patch makes sure we validate all possible scenarios.

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