linked-list: Order of insert_before/remove_at calls doesn't matter anymore

linked-list: Order of insert_before/remove_at calls doesn't matter anymore

This was quite confusing previously:  While calling insert_before()
and then remove_at() properly replaced the current item, calling them the
other way around inserted the new item before the previous item because
remove_at() changed the enumerator's position to the previous item.

The behavior in corner cases (calling the methods before or after
enumeration) is also changed slightly.