Fix handling of OID wraparound while in standalone mode.

If OID wraparound should occur while in standalone mode (unlikely but
possible), we want to advance the counter to FirstNormalObjectId not
FirstBootstrapObjectId.  Otherwise, user objects might be created with OIDs
in the system-reserved range.  That isn't immediately harmful but it poses
a risk of conflicts during future pg_upgrade operations.

Noted by Andres Freund.  Back-patch to all supported branches, since all of
them are supported sources for pg_upgrade operations.