pgsql: Support scrollable cursors (ie, 'direction' clause in FETCH) in

Log Message:
-----------
Support scrollable cursors (ie, 'direction' clause in FETCH) in plpgsql.
Pavel Stehule, reworked a bit by Tom.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        plpgsql.sgml (r1.106 -> r1.107)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plpgsql.sgml.diff?r1=1.106&r2=1.107)
    pgsql/src/backend/executor:
        spi.c (r1.176 -> r1.177)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/spi.c.diff?r1=1.176&r2=1.177)
    pgsql/src/include/executor:
        spi_priv.h (r1.28 -> r1.29)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/executor/spi_priv.h.diff?r1=1.28&r2=1.29)
    pgsql/src/pl/plpgsql/src:
        gram.y (r1.99 -> r1.100)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/gram.y.diff?r1=1.99&r2=1.100)
        pl_exec.c (r1.193 -> r1.194)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_exec.c.diff?r1=1.193&r2=1.194)
        plpgsql.h (r1.86 -> r1.87)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/plpgsql.h.diff?r1=1.86&r2=1.87)
        scan.l (r1.55 -> r1.56)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/scan.l.diff?r1=1.55&r2=1.56)
    pgsql/src/test/regress/expected:
        plpgsql.out (r1.51 -> r1.52)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/plpgsql.out.diff?r1=1.51&r2=1.52)
    pgsql/src/test/regress/sql:
        plpgsql.sql (r1.44 -> r1.45)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/plpgsql.sql.diff?r1=1.44&r2=1.45)