patch for type privileges

Here is the patch to implement type privileges that I alluded to
earlier. To recall, this is mainly so that owners can prevent others
from using their types because that would in some cases prevent owners
from changing the types. That would effectively be a denial of service.

These are the interfaces that this patch implements:

- GRANT USAGE ON DOMAIN
- GRANT USAGE ON TYPE
- default privileges for types
- analogous REVOKEs
- display privileges in psql \dT+
- privilege checks in various DDL commands (CREATE FUNCTION, CREATE
TABLE, etc.)
- various information schema views adjusted
- has_type_privilege function family

The basics here are mainly informed by the SQL standard. One thing from
there I did not implement is checking for permission of a type used in
CAST (foo AS type). This would be doable but relatively complicated,
and in practice someone how is not supposed to be able to use the type
wouldn't be able to create the cast or the underlying cast function
anyway for lack of access to the type.

As elsewhere in the system, the usage of TYPE and DOMAIN is partially
overlapping and partially not. You can use GRANT ON TYPE on a domain
but not GRANT ON DOMAIN on a type (compare CREATE/DROP). We only
support one common set of default privileges for types and domains. I
feel that's enough, but it could be adjusted.

Open items:

- GRANT TO ALL TYPES -- haven't gotten to that yet, but could be added

A reviewer should of course particularly check if there are any holes in
the privilege protection that this patch purports to afford.