Sender-based RPF and hot-root standby for ingress replication (IR) provider tunnels

This feature extends the sender-based RPF functionality for RSVP-P2MP, which, in conjunction with hot-root standby, provides support for live-live NGEN MVPN traffic. The configuration of the router, whether for RSVP-P2MP or IR provider tunnels, determines the form of sender-based RPF and hot-root standby that are implemented when their respective CLI configurations are enabled. Ingress replication works by introducing a unique VPN label to advertise each upstream PE router per VRF. This allows the IR to distinguish the sending PE router and the VRF. Note that when IR is used as the selective provider tunnel, IR tunnels must also be configured for all interested egress PE routers or border routers. When sender-based RPF is disabled, it causes all type 4 routes to be re-advertised with the VT/LSI label. IR is not intended to work in S-PMSI only configurations.