pgsql: Force pg_upgrade's to preserve pg_class.oid, not pg_class.relfil

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.

Branch
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master

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=2896c87ce4dc789722296f010eaefd5cbb86ece3

Modified Files
--------------
contrib/pg_upgrade/function.c | 10 ++--
contrib/pg_upgrade/info.c | 2 +-
contrib/pg_upgrade/pg_upgrade.c | 35 ++++++++-----
contrib/pg_upgrade/pg_upgrade.h | 4 +-
contrib/pg_upgrade_support/pg_upgrade_support.c | 63 +++++++++++++----------
src/backend/catalog/heap.c | 18 +++---
src/backend/catalog/index.c | 10 ++--
src/backend/catalog/toasting.c | 12 ++--
src/backend/commands/typecmds.c | 8 ++--
src/bin/pg_dump/pg_dump.c | 56 +++++++++------------
10 files changed, 112 insertions(+), 106 deletions(-)