Force pg_upgrade's to preserve pg_class.oid, not pg_class.relfilenode.

Bruce Momjian <bruce@momjian.us>

Commit: 2896c87ce4dc789722296f010eaefd5cbb86ece3
Author: Bruce Momjian <bruce@momjian.us>
Date: 2011-01-08T02:26:13Z
Releases: 9.1.0
Force pg_upgrade's to preserve pg_class.oid, not pg_class.relfilenode.
Toast tables have identical pg_class.oid and pg_class.relfilenode, but
for clarity it is good to preserve the pg_class.oid.

Update comments regarding what is preserved, and do some
variable/function renaming for clarity.

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