MEDIUM: list: Separate "locked" list from regular list.

Instead of using the same type for regular linked lists and "autolocked"
linked lists, use a separate type, "struct mt_list", for the autolocked one,
and introduce a set of macros, similar to the LIST_* macros, with the
MT_ prefix.
When we use the same entry for both regular list and autolocked list, as
is done for the "list" field in struct connection, we know have to explicitely
cast it to struct mt_list when using MT_ macros.
diff --git a/include/types/task.h b/include/types/task.h
index 3421cd3..481d563 100644
--- a/include/types/task.h
+++ b/include/types/task.h
@@ -118,7 +118,7 @@
  * TASK_WOKEN_OTHER and a context pointing to the work_list entry.
  */
 struct work_list {
-	struct list head;
+	struct mt_list head;
 	struct task *task;
 	void *arg;
 };