MINOR: lists: add LIST_ADDED() to check if an element belongs to a list

MINOR: lists: add LIST_ADDED() to check if an element belongs to a list

Some code parts use LIST_ISEMPTY() a lot on list elements to detect
if they were reset consecutive to their removal from a list, but this
test is always confusing as this was initially designed for list heads.

Instead let's have a new macro, LIST_ADDED(), which returns true when
the element is in a list (i.e. it's not "empty").