BFD: Improve handling of AdminDown

BFD: Improve handling of AdminDown

According to RFC 5882, system should not interpret the local or remote
session state transition to AdminDown as failure. We followed that for
the local session state but not for the remote session state (which
just triggered a transition of the local state to Down). The patch
fixes that.

We do not properly generate AdminDown on our side, so the patch is
relevant just for interoperability with other systems.

Thanks to Sunnat Samadov for the bugreport.