Adds new method for rule merge

IMPORTANT: SecDefaultAction specified on a child configuration will
overwrite the ones specified on the parent; Previously it was
concatenating.

src/actions/disruptive/drop.h

@@ -33,7 +33,7 @@ class Drop : public Action {
     explicit Drop(const std::string &action) : Action(action) { }
 
     bool evaluate(RuleWithActions *rule, Transaction *transaction,
-        std::shared_ptr<RuleMessage> rm) override;
+        RuleMessage &rm) override;
     bool isDisruptive() override { return true; }
 };