xtables-compat: add rule cache

xtables-compat: add rule cache

This patch adds a cache of rules within the nft handle. This feature is
useful since the whole ruleset was brought from the kernel for every
chain during listing operations. In addition with the new checks of
ruleset compatibility, the rule list is loaded one more time.

Now all the operations causing changes in the ruleset must invalidate
the cache, a function called flush_rule_cache has been introduced for
this purpose.

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