Show default privileges in information schema

Hitherto, the information schema only showed explicitly granted
privileges that were visible in the *acl catalog columns.  If no
privileges had been granted, the implicit privileges were not shown.

To fix that, add an SQL-accessible version of the acldefault()
function, and use that inside the aclexplode() calls to substitute the
catalog-specific default privilege set for null values.

reviewed by Abhijit Menon-Sen