nft-shared: support native tcp port delinearize

nft-shared: support native tcp port delinearize

This extends iptables-nft dissector to decode native tcp
port matching.  nft ruleset:

table ip filter {
        chain INPUT {
                type filter hook input priority filter; policy accept;
                tcp sport 12345
                tcp sport 12345 tcp dport 6789
                tcp sport < 1024
                tcp dport >= 1024
        }
}

$ iptables-nft-save
-A INPUT -p tcp -m tcp --sport 12345
-A INPUT -p tcp -m tcp --sport 12345 --dport 6789
-A INPUT -p tcp -m tcp --sport 0:1023
-A INPUT -p tcp -m tcp --dport 1024:65535

This would allow to extend iptables-nft to prefer
native payload expressions for --sport,dport in the future.

Also, parse_cmp must not clear the "payload" flag, this is because
cmp-based range expressions will contain following sequence:

payload => reg1
cmp reg1 > minv
cmp reg1 < maxv

... so second cmp would work.

Signed-off-by: Florian Westphal <fw@strlen.de>