pgsql: Fix a couple of planner bugs introduced by the new ability to

Log Message:
-----------
Fix a couple of planner bugs introduced by the new ability to discard
ORDER BY <constant> as redundant.  One is that this means query_planner()
has to canonicalize pathkeys even when the query jointree is empty;
the canonicalization was always a no-op in such cases before, but no more.
Also, we have to guard against thinking that a set-returning function is
"constant" for this purpose.  Add a couple of regression tests for these
evidently under-tested cases.  Per report from Greg Stark and subsequent
experimentation.

Modified Files:
--------------
    pgsql/src/backend/optimizer/path:
        equivclass.c (r1.2 -> r1.3)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/equivclass.c.diff?r1=1.2&r2=1.3)
    pgsql/src/backend/optimizer/plan:
        planmain.c (r1.101 -> r1.102)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/planmain.c.diff?r1=1.101&r2=1.102)
    pgsql/src/test/regress/expected:
        select.out (r1.17 -> r1.18)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/select.out.diff?r1=1.17&r2=1.18)
    pgsql/src/test/regress/sql:
        select.sql (r1.13 -> r1.14)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/select.sql.diff?r1=1.13&r2=1.14)