netfilter: nft_ct: add ct expectations support

This patch allows to add, list and delete expectations via nft objref
infrastructure and assigning these expectations via nft rule.

This allows manual port triggering when no helper is defined to manage a
specific protocol. For example, if I have an online game which protocol
is based on initial connection to TCP port 9753 of the server, and where
the server opens a connection to port 9876, I can set rules as follow:

table ip filter {
    ct expectation mygame {
        protocol udp;
        dport 9876;
        timeout 2m;
        size 1;
    }

    chain input {
        type filter hook input priority 0; policy drop;
        tcp dport 9753 ct expectation set "mygame";
    }

    chain output {
        type filter hook output priority 0; policy drop;
        udp dport 9876 ct status expected accept;
    }
}

Signed-off-by: Stéphane Veyret <sveyret@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>