Fix several problems in pg_dump's handling of SQL/MED objects, notably failure

to dump a PUBLIC user mapping correctly, as per bug #5560 from Shigeru Hanada.
Use the pg_user_mappings view rather than trying to access pg_user_mapping
directly, so that the code doesn't fail when run by a non-superuser.  And
clean up some minor carelessness such as unsafe usage of fmtId().

Back-patch to 8.4 where this code was added.