Add a role property 'rolinherit' which, when false, denotes that the role

doesn't automatically inherit the privileges of roles it is a member of;
for such a role, membership in another role can be exploited only by doing
explicit SET ROLE.  The default inherit setting is TRUE, so by default
the behavior doesn't change, but creating a user with NOINHERIT gives closer
adherence to our current reading of SQL99.  Documentation still lacking,
and I think the information schema needs another look.