*: Add and use option for graceful (re)start

*: Add and use option for graceful (re)start

Add a new start option "-K" to libfrr to denote a graceful start,
and use it in zebra and bgpd.

zebra will use this option to denote a planned FRR graceful restart
(supporting only bgpd currently) to wait for a route sync completion
from bgpd before cleaning up old stale routes from the FIB. An optional
timer provides an upper-bounds for this cleanup.

bgpd will use this option to denote either a planned FRR graceful
restart or a bgpd-only graceful restart, and this will drive the BGP
GR restarting router procedures.

Signed-off-by: Vivek Venkatraman <vivek@nvidia.com>