one last patch - array lower and upper bound

The "Allow easy display of usernames in a group (pg_hba.conf uses groups
now)" item on the open items, and subsequent plpgsql function I sent in,
made me realize it was too hard to get the upper and lower bound of an
array. The attached creates two functions that I think will be very
useful when combined with the ability of plpgsql to return sets.

array_lower(array, dim_num)
- and -
array_upper(array, dim_num)

They return the value (as an int) of the upper and lower bound of the
requested dim in the provided array.

With these, the show_group() function from before looks like:

CREATE OR REPLACE FUNCTION show_group(text) RETURNS SETOF text AS '
DECLARE
loginname text;
low int;
high int;
BEGIN
SELECT INTO low array_lower(grolist,1)
FROM pg_group WHERE groname = $1;
SELECT INTO high array_upper(grolist,1)
FROM pg_group WHERE groname = $1;

FOR i IN low..high LOOP
SELECT INTO loginname s.usename
FROM pg_shadow s join pg_group g on s.usesysid = g.grolist[i];
RETURN NEXT loginname;
END LOOP;
RETURN;
END;
' LANGUAGE 'plpgsql';

If possible, and no objections, please apply for 7.3.

catversion.h bump and initdb required.

Thanks,

Joe