xtables-compat: check if nft ruleset is compatible

xtables-compat: check if nft ruleset is compatible

This patch adds a verification of the compatibility between the nft
ruleset and iptables. Nft tables, chains and rules are checked to be
compatible with iptables. If something is not compatible, the execution
stops and an error message is displayed to the user.

This checking is triggered by xtables-compat -L and xtables-compat-save
commands.

Signed-off-by: Pablo M. Bermudo Garay <pablombg@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>