NSR for BGP family inet-vpn and inet6-vpn

This feature adds nonstop active routing (NSR) support for BGP family inet-vpn and family inet6-vpn. 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 BGP VPN 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 and can serve as a replacement for graceful restart.