Use GRANT system to manage access to sensitive functions

Use GRANT system to manage access to sensitive functions

Now that pg_dump will properly dump out any ACL changes made to
functions which exist in pg_catalog, switch to using the GRANT system
to manage access to those functions.

This means removing 'if (!superuser()) ereport()' checks from the
functions themselves and then REVOKEing EXECUTE right from 'public' for
these functions in system_views.sql.

Reviews by Alexander Korotkov, Jose Luis Tallon