NG-MVPN: PIM Source-Specific Multicast (SSM)

This feature adds support for PIM source-specific multicast (PIM SSM) for next-generation MVPN. PIM source-specific multicast uses a subset of PIM sparse mode and IGMP version 3 (IGMPv3) to allow a client to receive multicast traffic directly from the source. PIM source-specific multicast uses the PIM sparse-mode functionality to create a shortest-path tree (SPT) between the receiver and the source but builds the SPT without the help of a rendezvous point (RP).