This patch adds support for a new BFD session down message from zebra to

This patch adds support for a new BFD session down message from zebra to

protocols. BGP and OSPF are integrated to respond this BFD session down message
originated in Zebra via ptmd.

BGP and OSPF now have a bfd command, which tells OSPF/BGP to respond to the
BFD session down message.

OSPF:

interface <>
 ip ospf bfd

BGP:

router bgp <>
  neighbor <> bfd

Please note that these commands don't enable BFD as a protocol. BFD configuration
and paramter tuning are via BFD applicable UI.

Signed-off-by: Vipin Kumar <vipin@cumulusnetworks.com>
Reviewed-by: Shrijeet Mukherjee <shm@cumulusnetworks.com>