Change PROCEDURE to FUNCTION in CREATE TRIGGER syntax

Change PROCEDURE to FUNCTION in CREATE TRIGGER syntax

Since procedures are now a different thing from functions, change the
CREATE TRIGGER and CREATE EVENT TRIGGER syntax to use FUNCTION in the
clause that specifies the function.  PROCEDURE is still accepted for
compatibility.

pg_dump and ruleutils.c output is not changed yet, because that would
require a change in information_schema.sql and thus a catversion change.

Reported-by: Peter Geoghegan <pg@bowt.ie>
Reviewed-by: Jonathan S. Katz <jonathan.katz@excoventures.com>