zebra - BFD client de-registration support

zebra - BFD client de-registration support

CM-10680
Issue: When BGP daemon is stopped, all the BGP BFD sessions are not getting deleted from PTM.
Root cause: BGP daemon stop causes BFD de-register message to be sent for every peer on which BFD is enabled. But, all the de-register messages from bgpd to zebra are not processed before the socket close. This results in some stale BGP BFD sessions.
Fix: Support for client de-register message has been added in PTM/BFD.  Changes in Quagga to support BFD client de-registrations:
−   The BFD clients de-registration is sent directly from zebra daemon when zebra client (bgpd, ospfd and ospf6d) socket close is detected.
−   Introduced a BFD flag for the zebra clients to prevent BFD de-registration messages from being sent to zebra daemon when the client is shutting down. This reduces the BFD messaging.

CM-10540
Issue: Invalid ptm status “fail” instead of “n/a” being displayed for VRF interfaces.
Root cause: ptm status is not being initialized to “unknown” status when VRF interface is added or changed. The uninitialized value is ‘0’ which is the value for “fail”
Fix: Initialized the ptm status to the correct value.

Signed-off-by: Radhika Mahankali <radhika@cumulusnetworks.com>
Reviewed-by: Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by: Kanna Rajagopal <kanna@cumulusnetworks.com>
Ticket: CM-10680, CM-10540
Reviewed By: CCR-4653
Testing Done: PTM smoke, BGP smoke and ptmd_test.py:TestMultipleAddrsIntfOspfBgp