Improve ilist.h's support for deletion of slist elements during iteration.

Previously one had to use slist_delete(), implying an additional scan of
the list, making this infrastructure considerably less efficient than
traditional Lists when deletion of element(s) in a long list is needed.
Modify the slist_foreach_modify() macro to support deleting the current
element in O(1) time, by keeping a "prev" pointer in addition to "cur"
and "next".  Although this makes iteration with this macro a bit slower,
no real harm is done, since in any scenario where you're not going to
delete the current list element you might as well just use slist_foreach
instead.  Improve the comments about when to use each macro.

Back-patch to 9.3 so that we'll have consistent semantics in all branches
that provide ilist.h.  Note this is an ABI break for callers of
slist_foreach_modify().

Andres Freund and Tom Lane

src/backend/lib/ilist.c

@@ -28,7 +28,7 @@
  *
  * It is not allowed to delete a 'node' which is is not in the list 'head'
  *
- * Caution: this is O(n)
+ * Caution: this is O(n); consider using slist_delete_current() instead.
  */
 void
 slist_delete(slist_head *head, slist_node *node)