src: allow to use variables in flowtable and chain devices

src: allow to use variables in flowtable and chain devices

This patch adds support for using variables for devices in the chain and
flowtable definitions, eg.

 define if_main = lo

 table netdev filter1 {
    chain Main_Ingress1 {
        type filter hook ingress device $if_main priority -500; policy accept;
    }
 }

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