ospfd, ospf6d: introduce the "graceful-restart hello-delay" command

ospfd, ospf6d: introduce the "graceful-restart hello-delay" command

This command makes unplanned GR more reliable by manipulating the
sending of Grace-LSAs and Hello packets for a certain amount of time,
increasing the chance that the neighboring routers are aware of
the ongoing graceful restart before resuming normal OSPF operation.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>