inspection of shared catalogs.  This allows pg_dumpall to continue to
work with pre-8.1 servers that likely won't have a database named postgres.
Also, suppress output of SYSID options for users and groups, since server
no longer does anything with these except emit a rude message.
There is much more to be done to update pg_dumpall for the roles feature,
but this at least makes it usable again.  Per gripe from Chris K-L.