*: Convert list_delete(struct list *) to ** to allow nulling

*: Convert list_delete(struct list *) to ** to allow nulling

Convert the list_delete(struct list *) function to use
struct list **.  This is to allow the list pointer to be nulled.

I keep running into uses of this list_delete function where we
forget to set the returned pointer to NULL and attempt to use
it and then experience a crash, usually after the developer
has long since left the building.

Let's make the api explicit in it setting the list pointer
to null.

Cynical Prediction:  This code will expose a attempt
to use the NULL'ed list pointer in some obscure bit
of code.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>