variadic function support

Hello

this patch enhance current syntax of CREATE FUNCTION statement. It
allows creating functions with variable number of arguments. This
version is different than last my patches. It doesn't need patching
PL. Basic idea is transformation of real arguments (related to
declared variadic argument) to array. All changes are mostly in
parser.

Demo:
CREATE FUNCTION public.least(double precision[]) RETURNS double precision AS $$
SELECT min($1[i])
FROM generate_subscripts($1,1) g(i)
$$ LANGUAGE SQL VARIADIC;

SELECT public.least(3,2,1);
least
-------
1
(1 row)

SELECT public.least(3,2,1,0,-1);
least
-------
-1
CREATE FUNCTION concat(varchar, anyarray) RETURNS varchar AS $$
SELECT array_to_string($2, $1);
$$ LANGUAGE SQL VARIADIC;

SELECT concat('-',2008,10,12);
concat
------------
2008-10-12
(1 row)

Regards
Pavel Stehule