Shore up ADMIN OPTION restrictions.

Shore up ADMIN OPTION restrictions.

Granting a role without ADMIN OPTION is supposed to prevent the grantee
from adding or removing members from the granted role.  Issuing SET ROLE
before the GRANT bypassed that, because the role itself had an implicit
right to add or remove members.  Plug that hole by recognizing that
implicit right only when the session user matches the current role.
Additionally, do not recognize it during a security-restricted operation
or during execution of a SECURITY DEFINER function.  The restriction on
SECURITY DEFINER is not security-critical.  However, it seems best for a
user testing his own SECURITY DEFINER function to see the same behavior
others will see.  Back-patch to 8.4 (all supported versions).

The SQL standards do not conflate roles and users as PostgreSQL does;
only SQL roles have members, and only SQL users initiate sessions.  An
application using PostgreSQL users and roles as SQL users and roles will
never attempt to grant membership in the role that is the session user,
so the implicit right to add or remove members will never arise.

The security impact was mostly that a role member could revoke access
from others, contrary to the wishes of his own grantor.  Unapproved role
member additions are less notable, because the member can still largely
achieve that by creating a view or a SECURITY DEFINER function.

Reviewed by Andres Freund and Tom Lane.  Reported, independently, by
Jonas Sundman and Noah Misch.

Security: CVE-2014-0060