Fix infer_arbiter_indexes() to not barf on system columns.

Fix infer_arbiter_indexes() to not barf on system columns.

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.