GRES: Link fault management

The Ethernet link fault management daemon (lfmd) runs on the backup Routing Engine when GRES is configured. When the lfmd daemon runs on the backup Routing Engine as well, the link fault management (LFM) states are kept in sync and so minimal work is required by the lfmd daemon after the switchover. 

To verify that the LFM states are in sync, use the show oam ethernet link-fault-management command on both primary and backup Routing Engines. The lfmd daemon runs only on the primary Routing Engine when GRES is not configured.