pgsql: Support EXECUTE USING in plpgsql.

Log Message:
-----------
Support EXECUTE USING in plpgsql.

Pavel Stehule, with some improvements by myself.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        plpgsql.sgml (r1.125 -> r1.126)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plpgsql.sgml?r1=1.125&r2=1.126)
    pgsql/src/pl/plpgsql/src:
        gram.y (r1.108 -> r1.109)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/gram.y?r1=1.108&r2=1.109)
        pl_exec.c (r1.207 -> r1.208)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_exec.c?r1=1.207&r2=1.208)
        pl_funcs.c (r1.67 -> r1.68)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_funcs.c?r1=1.67&r2=1.68)
        plpgsql.h (r1.95 -> r1.96)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/plpgsql.h?r1=1.95&r2=1.96)
        scan.l (r1.60 -> r1.61)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/scan.l?r1=1.60&r2=1.61)
    pgsql/src/test/regress/expected:
        plpgsql.out (r1.59 -> r1.60)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/plpgsql.out?r1=1.59&r2=1.60)
    pgsql/src/test/regress/sql:
        plpgsql.sql (r1.49 -> r1.50)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/plpgsql.sql?r1=1.49&r2=1.50)