Make ALTER DEFAULT PRIVILEGES require privileges, not membership.

If role A is a direct or indirect member of role B but does not inherit
B's privileges (because at least one relevant grant was created WITH
INHERIT FALSE) then A should not be permitted to bypass privilege
checks that require the privileges of B. For example, A can't change
the privileges of objects owned by B, nor can A drop those objects.

However, up until now, it's been possible for A to change default
privileges for role B. That doesn't seem to be correct, because a
non-inherited role grant is only supposed to permit you to assume
the identity of the granted role via SET ROLE, and should not
otherwise permit you to exercise the privileges of that role. Most
places followed that rule, but this case was an exception.

This could be construed as a security vulnerability, but it does not
seem entirely clear cut, since older branches were fuzzy about the
distinction between is_member_of_role() and has_privs_of_role() in
a number of other ways as well. Because of this, and because
user-visible behavior changes in minor releases are to be avoided
whenever possible, no back-patch.

Discussion: http://postgr.es/m/CA+TgmobG_YUP06R_PM_2Z7wR0qv_52gQPHD8CYXbJva0cf0E+A@mail.gmail.com

src/backend/catalog/aclchk.c

@@ -1117,13 +1117,10 @@ ExecAlterDefaultPrivilegesStmt(ParseState *pstate, AlterDefaultPrivilegesStmt *s
 
 			iacls.roleid = get_rolespec_oid(rolespec, false);
 
-			/*
-			 * We insist that calling user be a member of each target role. If
-			 * he has that, he could become that role anyway via SET ROLE, so
-			 * FOR ROLE is just a syntactic convenience and doesn't give any
-			 * special privileges.
-			 */
-			check_is_member_of_role(GetUserId(), iacls.roleid);
+			if (!has_privs_of_role(GetUserId(), iacls.roleid))
+				ereport(ERROR,
+						(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+						 errmsg("permission denied to change default privileges")));
 
 			SetDefaultACLsInSchemas(&iacls, nspnames);
 		}