pgsql: Require the issuer of CREATE TYPE to own the functions mentioned

Log Message:
-----------
Require the issuer of CREATE TYPE to own the functions mentioned in the
type definition. Because use of a type's I/O conversion functions isn't
access-checked, CREATE TYPE amounts to granting public execute permissions
on the functions, and so allowing it to anybody means that someone could
theoretically gain access to a function he's not supposed to be able to
execute. The parameter-type restrictions already enforced by CREATE TYPE
make it fairly unlikely that this oversight is meaningful in practice,
but still it seems like a good idea to plug the hole going forward.
Also, document the implicit grant just in case anybody gets the idea of
building I/O functions that might need security restrictions.

Modified Files:
--------------
pgsql/doc/src/sgml/ref:
create_type.sgml (r1.59 -> r1.60)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/create_type.sgml.diff?r1=1.59&r2=1.60)
pgsql/src/backend/commands:
typecmds.c (r1.85 -> r1.86)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/typecmds.c.diff?r1=1.85&r2=1.86)