+/*
+ * key != NULL && value == NULL
+ * remove node with key 'key', equivalent to
+ * config_node_remove(rec, parent, config_node_find(parent, key))
+ * key == NULL && value != NULL
+ * create a new node with type NODE_TYPE_VALUE and value 'value'
+ * key != NULL && value != NULL
+ * if a node with key 'key' exists change its value to 'value',
+ * otherwise create a new node with type NODE_TYPE_KEY, key 'key' and value 'value'
+ * */