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.

examples/simple_example_using_c/test.c

@@ -68,6 +68,8 @@ int main (int argc, char **argv)
     msc_process_response_body(transaction);
     msc_process_logging(transaction);
 end:
+    if(error != NULL)
+        msc_rules_error_cleanup(error);
     msc_rules_cleanup(rules);
     msc_cleanup(modsec);