There are some bugs about backward scanning using

indexes.

1. Index Scan using plural indexids never scan backward
   as to the order of indexids.
2. The cursor using Index scan is not usable after moving
   past the end.

This patch solves above bugs.
Moreover the change of _bt_first() would be useful to extend
ORDER BY patch by Jan Wieck for all descending order cases.

Hiroshi Inoue