feat(ll): provide a ds_*ll_init func for consistency

ds.c

@@ -3,6 +3,18 @@
 
 #include "ds.h"
 
+ds_sll_t
+*ds_sll_init(void)
+{
+	return ds_sll_new_node(NULL);
+}
+
+ds_dll_t
+*ds_dll_init(void)
+{
+	return ds_dll_new_node(NULL);
+}
+
 ds_sll_t
 *ds_sll_new_node(void *data)
 {