Add Bfd diagnostic code for when neighbor signaled session down.

[AnilKR123]

Change Scope

I have added a Bfd diagnostic code for when neighbor signals session down. This is specified in RFC5880 but is absent from the public model.

From RFC5880:

 A diagnostic code specifying the local system's reason for the
  last change in session state.  Values are:

     0 -- No Diagnostic
     1 -- Control Detection Time Expired
     2 -- Echo Function Failed
     3 -- Neighbor Signaled Session Down
     4 -- Forwarding Plane Reset
     5 -- Path Down
     6 -- Concatenated Path Down
     7 -- Administratively Down
     8 -- Reverse Concatenated Path Down

[dplore]

To keep this as a non-breaking change, the original ordering of the enums must be preserved. You can only add enums to the list.

[dplore]

I realize the enum value won't match the value in a packet on the wire, but that is not required.

[AnilKR123]

I moved NEIGHBOR_DOWN to the end of the enums list.

Thanks.