pgsql: Add BY clause to PL/PgSQL FOR loop, to control the iteration

Log Message:
-----------
Add BY clause to PL/PgSQL FOR loop, to control the iteration increment.

Jaime Casanova

Modified Files:
--------------
    pgsql/doc/src/sgml:
        plpgsql.sgml (r1.94 -> r1.95)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plpgsql.sgml.diff?r1=1.94&r2=1.95)
    pgsql/src/pl/plpgsql/src:
        gram.y (r1.90 -> r1.91)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/gram.y.diff?r1=1.90&r2=1.91)
        pl_exec.c (r1.169 -> r1.170)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_exec.c.diff?r1=1.169&r2=1.170)
        pl_funcs.c (r1.52 -> r1.53)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_funcs.c.diff?r1=1.52&r2=1.53)
        plpgsql.h (r1.74 -> r1.75)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/plpgsql.h.diff?r1=1.74&r2=1.75)
        scan.l (r1.49 -> r1.50)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/scan.l.diff?r1=1.49&r2=1.50)