While it could be argued that rejecting system column mentions in the
ON CONFLICT list is an unsupported feature, falling over altogether
just because the table has a unique index on OID is indubitably a bug.

As far as I can tell, fixing infer_arbiter_indexes() is sufficient to
make ON CONFLICT (oid) actually work, though making a regression test
for that case is problematic because of the impossibility of setting
the OID counter to a known value.

Minor cosmetic cleanups along with the bug fix.