src/utils_llist.[ch]: Changed the semantic so module doesn't copy the `key'.

src/utils_llist.[ch]: Changed the semantic so module doesn't copy the `key'.

Whilst looking at my implementation of the plugins we've discussed which are
now using dual avl trees, I saw that the utils_llist module would be a more
efficient substitute for the second avl tree. However, it would need to know
its size and would preferably not duplicate and own the key for each item.
Here's a patch which does this. These changes might make it easier for future
plugins to use the linked list for similar purposes, similar to the way the avl
tree does not impose any lifetime on its keys.

Signed-off-by: Florian Forster <octo@huhu.verplant.org>