Match pg_user_mappings limits to information_schema.user_mapping_options.

Both views replace the umoptions field with NULL when the user does not
meet qualifications to see it.  They used different qualifications, and
pg_user_mappings documented qualifications did not match its implemented
qualifications.  Make its documentation and implementation match those
of user_mapping_options.  One might argue for stronger qualifications,
but these have long, documented tenure.  pg_user_mappings has always
exhibited this problem, so back-patch to 9.2 (all supported versions).

Michael Paquier and Feike Steenbergen.  Reviewed by Jeff Janes.
Reported by Andrew Wheelwright.

Security: CVE-2017-7486

src/backend/catalog/system_views.sql

@@ -669,11 +669,11 @@ CREATE VIEW pg_user_mappings AS
         ELSE
             A.rolname
         END AS usename,
-        CASE WHEN pg_has_role(S.srvowner, 'USAGE') OR has_server_privilege(S.oid, 'USAGE') THEN
-            U.umoptions
-        ELSE
-            NULL
-        END AS umoptions
+        CASE WHEN (U.umuser <> 0 AND A.rolname = current_user)
+                    OR (U.umuser = 0 AND pg_has_role(S.srvowner, 'USAGE'))
+                    OR (SELECT rolsuper FROM pg_authid WHERE rolname = current_user)
+                    THEN U.umoptions
+                 ELSE NULL END AS umoptions
     FROM pg_user_mapping U
          LEFT JOIN pg_authid A ON (A.oid = U.umuser) JOIN
         pg_foreign_server S ON (U.umserver = S.oid);