Constify the arguments of ilist.c/h functions

Constify the arguments of ilist.c/h functions

Const qualifiers ensure that we don't do something stupid in the
function implementation.  Additionally they clarify the interface.  As
an example:

    void
    slist_delete(slist_head *head, const slist_node *node)

Here one can instantly tell that node->next is not going to be set to
NULL.  Finally, const qualifiers potentially allow the compiler to do
more optimizations.  This being said, no benchmarking was done for
this .

The functions that return non-const pointers like slist_next_node(),
dclist_next_node() etc. are not affected by the  intentionally.

Author: Aleksander Alekseev
Reviewed-by: Andres Freund
Discussion: https://postgr.es/m/CAJ7c6TM2%3D08mNKD9aJg8vEY9hd%2BG4L7%2BNvh30UiNT3kShgRgNg%40mail.gmail.com