WL#5781: DEFAULT/catch-all partition for LIST partitioning

Add DEFAULT partition for LIST partitioning (like MAXVALUE for RANGE partitioning)
as a catch-all partition for rows not covered in any other partition.

CREATE TABLE t (a INT, b TEXT)
PARTITION BY LIST (a)
(PARTITION p0 VALUES IN (1,5,6,7),
 PARTITION p1 VALUES IN (3,4,9),
 PARTITION pDefault VALUES IN (DEFAULT));

When using a DEFAULT partition, it will not longer be possible to ADD a
partition, but one must instead REORGANIZE the DEFAULT partition, just like
MAXVALUE partition in RANGE partitioning.

See BUG#50566.