optimize: merge rules with same selectors into a concatenation

optimize: merge rules with same selectors into a concatenation

This patch extends the ruleset optimization infrastructure to collapse
several rules with the same selectors into a concatenation.

Transform:

  meta iifname eth1 ip saddr 1.1.1.1 ip daddr 2.2.2.3 accept
  meta iifname eth1 ip saddr 1.1.1.2 ip daddr 2.2.2.5 accept
  meta iifname eth2 ip saddr 1.1.1.3 ip daddr 2.2.2.6 accept

into:

  meta iifname . ip saddr . ip daddr { eth1 . 1.1.1.1 . 2.2.2.6, eth1 . 1.1.1.2 . 2.2.2.5 , eth1 . 1.1.1.3 . 2.2.2.6 } accept

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