pgsql: Fix column privilege checking for cases where parent and child

Log Message:
-----------
Fix column privilege checking for cases where parent and child have different
attribute numbering. Also, a parent whole-row reference should not require
select privilege on child columns that aren't inherited from the parent.
Problem diagnosed by KaiGai Kohei, though this isn't exactly his patch.

Modified Files:
--------------
pgsql/src/backend/optimizer/prep:
prepunion.c (r1.166 -> r1.167)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/prep/prepunion.c?r1=1.166&r2=1.167)
pgsql/src/test/regress/expected:
privileges.out (r1.45 -> r1.46)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/privileges.out?r1=1.45&r2=1.46)
pgsql/src/test/regress/sql:
privileges.sql (r1.24 -> r1.25)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/privileges.sql?r1=1.24&r2=1.25)