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.

headers/modsecurity/rules_set.h

@@ -99,6 +99,7 @@ int msc_rules_add_remote(RulesSet *rules, const char *key, const char *uri,
     const char **error);
 int msc_rules_add_file(RulesSet *rules, const char *file, const char **error);
 int msc_rules_add(RulesSet *rules, const char *plain_rules, const char **error);
+void msc_rules_error_cleanup(const char *error);
 int msc_rules_cleanup(RulesSet *rules);
 
 #ifdef __cplusplus

headers/modsecurity/transaction.h

@@ -723,6 +723,9 @@ void msc_transaction_cleanup(Transaction *transaction);
 /** @ingroup ModSecurity_C_API */
 int msc_intervention(Transaction *transaction, ModSecurityIntervention *it);
 
+/** @ingroup ModSecurity_C_API */
+void msc_intervention_cleanup(ModSecurityIntervention *it);
+
 /** @ingroup ModSecurity_C_API */
 int msc_process_logging(Transaction *transaction);