src: add --define key=value

src: add --define key=value

This patch adds a new option to define variables from the command line.

 # cat test.nft
 table netdev x {
        chain y {
                type filter hook ingress devices = $dev priority 0;
                counter accept
        }
 }
 # nft --define dev="{ eth0, eth1 }" -f test.nft

You can only combine it with -f/--filename.

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