NSR for BGP-LU family inet and inet6

This feature adds nonstop active routing (NSR) support for BGP labeled-unicast (BGP-LU) family inet and family inet6. NSR uses the same infrastructure as graceful Routing Engine switchover (GRES) to preserve interface and kernel information, while also preserving routing protocol state by running the routing protocol process (rpd) on the backup Routing Engine. This allows IPv4 and IPv6 BGP labeled-unicast routing information and protocol state to be maintained across a Routing Engine switchover without relying on neighboring routers to support graceful restart extensions. Because NSR is self-contained, it is particularly useful in networks where neighboring routers do not support graceful restart.