xtables: Implement per chain rule cache

xtables: Implement per chain rule cache

Use recently introduced support for rules inside chains in libnftnl to
introduce a rule cache per chain instead of a global one.

A tricky bit is to decide if cache should be updated or not. Previously,
the global rule cache was populated just once and then reused unless
being flushed completely (via call to flush_rule_cache() with
NULL-pointer table argument). Resemble this behaviour by introducing a
boolean indicating cache status and fetch rules for all chains when
updating the chain cache in nft_chain_list_get().

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>