Initial BFD commit, work in progress.

lib/lists.c

@@ -100,6 +100,27 @@ rem_node(node *n)
   x->prev = z;
 }
 
+/**
+ * replace_node - replace a node in a list with another one
+ * @old: node to be removed
+ * @new: node to be inserted
+ *
+ * Replaces node @old in the list it's linked in with node @new.  Node
+ * @old may be a copy of the original node, which is not accessed
+ * through the list. The function could be called with @old == @new,
+ * which just fixes neighbors' pointers in the case that the node
+ * was reallocated.
+ */
+LIST_INLINE void
+replace_node(node *old, node *new)
+{
+  old->next->prev = new;
+  old->prev->next = new;
+
+  new->prev = old->prev;
+  new->next = old->next;
+}
+
 /**
  * init_list - create an empty list
  * @l: list