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/msg.h

@@ -41,10 +41,10 @@ class Msg : public Action {
             m_string(std::move(z)) { }
 
     bool evaluate(RuleWithActions *rule, Transaction *transaction,
-        std::shared_ptr<RuleMessage> rm) override;
+        RuleMessage &rm) override;
 
     std::string data(Transaction *Transaction);
-    std::unique_ptr<RunTimeString> m_string;
+    std::shared_ptr<RunTimeString> m_string;
 };