pgsql: Clean up some residual confusion between OIDs and RelFileNumbers

Clean up some residual confusion between OIDs and RelFileNumbers.

Commit b0a55e43299c4ea2a9a8c757f9c26352407d0ccc missed a few places
where we are referring to the number used as a part of the relation
filename as an "OID". We now want to call that a "RelFileNumber".

Some of these places actually made it sound like the OID in question
is pg_class.oid rather than pg_class.relfilenode, which is especially
good to clean up.

Dilip Kumar with some editing by me.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/851f4cc75cdd8c831f1baa9a7abf8c8248b65890

Modified Files
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src/backend/commands/dbcommands.c | 4 ++--
src/backend/replication/basebackup.c | 16 +++++++------
src/backend/storage/file/reinit.c | 44 ++++++++++++++++++------------------
src/include/storage/reinit.h | 3 ++-
4 files changed, 35 insertions(+), 32 deletions(-)