extended credential_set_t interface by a cache_cert() method

allows persistent or in-memory caching of fetched certificates

src/libstrongswan/utils.c

@@ -71,6 +71,13 @@ void *return_null()
 	return NULL;
 }
 
+/**
+ * nop operation
+ */
+void nop()
+{
+}
+
 /**
  * We use a single mutex for all refcount variables. This
  * is not optimal for performance, but the critical section