pgsql: Allow the second argument of pg_get_expr() to be just zero when

Log Message:
-----------
Allow the second argument of pg_get_expr() to be just zero when deparsing
an expression that's not supposed to contain variables.  Per discussion
with Gevik Babakhani, this eliminates the need for an ugly kluge (namely,
specifying some unrelated relation name).  Remove one such kluge from
pg_dump.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        func.sgml (r1.480 -> r1.481)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml?r1=1.480&r2=1.481)
    pgsql/src/backend/utils/adt:
        ruleutils.c (r1.297 -> r1.298)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/ruleutils.c?r1=1.297&r2=1.298)
    pgsql/src/bin/pg_dump:
        pg_dump.c (r1.536 -> r1.537)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_dump.c?r1=1.536&r2=1.537)