pgsql: Fix up btree's initial scankey processing to be able to detect

Log Message:
-----------
Fix up btree's initial scankey processing to be able to detect redundant
or contradictory keys even in cross-data-type scenarios.  This is another
benefit of the opfamily rewrite: we can find the needed comparison
operators now.

Modified Files:
--------------
    pgsql/src/backend/access/nbtree:
        README (r1.14 -> r1.15)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/README.diff?r1=1.14&r2=1.15)
        nbtsearch.c (r1.108 -> r1.109)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtsearch.c.diff?r1=1.108&r2=1.109)
        nbtutils.c (r1.79 -> r1.80)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtutils.c.diff?r1=1.79&r2=1.80)