src: Support intra-transaction rule references

src: Support intra-transaction rule references

A rule may be added before or after another one using index keyword. To
support for the other rule being added within the same batch, one has to
make use of NFTNL_RULE_ID and NFTNL_RULE_POSITION_ID attributes. This
patch does just that among a few more crucial things:

* If cache is complete enough to contain rules, update cache when
  evaluating rule commands so later index references resolve correctly.

* Reduce rule_translate_index() to its core code which is the actual
  linking of rules and consequently rename the function. The removed
  bits are pulled into the calling rule_evaluate() to reduce code
  duplication in between cache updates with and without rule reference.

* Pass the current command op to rule_evaluate() as indicator whether to
  insert before or after a referenced rule or at beginning or end of
  chain in cache. Exploit this from chain_evaluate() to avoid adding
  the chain's rules a second time.

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