patch 8.1.0614: placing signs can be complicated

Problem:    Placing signs can be complicated.
Solution:   Add functions for defining and placing signs.  Introduce a group
            name to avoid different plugins using the same signs. (Yegappan
            Lakshmanan, closes #3652)

src/list.c

@@ -115,6 +115,20 @@ rettv_list_alloc(typval_T *rettv)
     return OK;
 }
 
+/*
+ * Same as rettv_list_alloc() but uses an allocation id for testing.
+ */
+    int
+rettv_list_alloc_id(typval_T *rettv, alloc_id_T id UNUSED)
+{
+#ifdef FEAT_EVAL
+    if (alloc_fail_id == id && alloc_does_fail((long_u)sizeof(list_T)))
+	return FAIL;
+#endif
+    return rettv_list_alloc(rettv);
+}
+
+
 /*
  * Set a list as the return value
  */