Added methods to free buffers allocated by ModSecurity APIs

- The following methods are introduced to allow clients of
  libModSecurity that are not able to link and call the C/C++ standard
  library to be able to free the buffers allocated by libModSecurity.
- msc_intervention_cleanup: Frees the buffers in a
  ModSecurityIntervention structure that have been allocated by calls to
  msc_intervention.
- msc_rules_error_cleanup: Frees an error message buffer allocated by
  the msc_rules_xxx functions to detail the condition that triggered
  the error.

src/rules_set.cc

@@ -311,6 +311,21 @@ extern "C" int msc_rules_add(RulesSet *rules, const char *plain_rules,
 }
 
 
+/**
+ * @name    msc_rules_error_cleanup
+ * @brief   Deallocates an error message buffer returned by a msc_rules_xxx function.
+ *
+ * This is a helper function to free the error message buffer allocated
+ * by a msc_rules_xxx function.
+ *
+ * @param error Error message pointer.
+ *
+ */
+extern "C" void msc_rules_error_cleanup(const char *error) {
+    free((void*) error);
+}
+
+
 extern "C" int msc_rules_cleanup(RulesSet *rules) {
     delete rules;
     return true;

src/transaction.cc

@@ -2273,6 +2273,22 @@ extern "C" int msc_intervention(Transaction *transaction,
 }
 
 
+/**
+ * @name    msc_intervention_cleanup
+ * @brief   Removes all the resources allocated by a given Intervention.
+ *
+ * This is a helper function to free any allocated buffers owned by the
+ * intervention.
+ *
+ * @param it ModSecurity intervention.
+ *
+ */
+extern "C" void msc_intervention_cleanup(ModSecurityIntervention *it) {
+    intervention::free(it);
+    intervention::clean(it);
+}
+
+
 /**
  * @name    msc_get_response_body
  * @brief   Retrieve a buffer with the updated response body.