MINOR: list: add a macro to detect that a list contains at most one element

MINOR: list: add a macro to detect that a list contains at most one element

The new LIST_ATMOST1() test verifies that the designated element is either
alone or points on both sides to the same element. This is used to detect
that a list has at most a single element, or that an element about to be
deleted was the last one of a list.