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