pgsql: Repair two constraint-exclusion corner cases triggered by proving

Log Message:
-----------
Repair two constraint-exclusion corner cases triggered by proving that an
inheritance child of an UPDATE/DELETE target relation can be excluded by
constraints.  I had rearranged some code in set_append_rel_pathlist() to
avoid "useless" work when a child is excluded, but overdid it and left
the child with no cheapest_path entry, causing possible failure later
if the appendrel was involved in a join.  Also, it seems that the dummy
plan generated by inheritance_planner() when all branches are excluded
has to be a bit less dummy now than was required in 8.2.
Per report from Jan Wieck.  Add his test case to the regression tests.

Modified Files:
--------------
    pgsql/src/backend/optimizer/path:
        allpaths.c (r1.163 -> r1.164)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/allpaths.c.diff?r1=1.163&r2=1.164)
    pgsql/src/backend/optimizer/plan:
        planner.c (r1.220 -> r1.221)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/planner.c.diff?r1=1.220&r2=1.221)
    pgsql/src/test/regress/expected:
        rules.out (r1.127 -> r1.128)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/rules.out.diff?r1=1.127&r2=1.128)
    pgsql/src/test/regress/sql:
        rules.sql (r1.33 -> r1.34)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/rules.sql.diff?r1=1.33&r2=1.34)