pgsql: Implement RETURN QUERY for PL/PgSQL.

Log Message:
-----------
Implement RETURN QUERY for PL/PgSQL. This provides some convenient syntax
sugar for PL/PgSQL set-returning functions that want to return the result
of evaluating a query; it should also be more efficient than repeated
RETURN NEXT statements. Based on an earlier patch from Pavel Stehule.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        plpgsql.sgml (r1.115 -> r1.116)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plpgsql.sgml.diff?r1=1.115&r2=1.116)
    pgsql/src/pl/plpgsql/src:
        gram.y (r1.104 -> r1.105)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/gram.y.diff?r1=1.104&r2=1.105)
        pl_exec.c (r1.198 -> r1.199)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_exec.c.diff?r1=1.198&r2=1.199)
        pl_funcs.c (r1.62 -> r1.63)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_funcs.c.diff?r1=1.62&r2=1.63)
        plpgsql.h (r1.90 -> r1.91)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/plpgsql.h.diff?r1=1.90&r2=1.91)
        scan.l (r1.57 -> r1.58)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/scan.l.diff?r1=1.57&r2=1.58)
    pgsql/src/test/regress/expected:
        plpgsql.out (r1.57 -> r1.58)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/plpgsql.out.diff?r1=1.57&r2=1.58)
    pgsql/src/test/regress/sql:
        plpgsql.sql (r1.48 -> r1.49)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/plpgsql.sql.diff?r1=1.48&r2=1.49)