pimd: PIM Bootstrap packet processing

pimd: PIM Bootstrap packet processing

1. Packet validation as per RFC 5059 Sec 3.1.3
We won't supporting scope zone BSM as of now, they are dropped now.
Order of the check slightly be changed in code for optimization.

   if ((DirectlyConnected(BSM.src_ip_address) == FALSE) OR
        (we have no Hello state for BSM.src_ip_address)) {
     drop the Bootstrap message silently
   }

   if (BSM.dst_ip_address == ALL-PIM-ROUTERS) {
     if (BSM.no_forward_bit == 0) {
       if (BSM.src_ip_address != RPF_neighbor(BSM.BSR_ip_address)) {
         drop the Bootstrap message silently
       }
     } else if ((any previous BSM for this scope has been accepted) OR
                (more than BS_Period has elapsed since startup)) {
       #only accept no-forward BSM if quick refresh on startup
       drop the Bootstrap message silently
     }
   } else if ((Unicast BSM support enabled) AND
              (BSM.dst_ip_address is one of my addresses)) {
     if ((any previous BSM for this scope has been accepted) OR
         (more than BS_Period has elapsed since startup)) {
       #the packet was unicast, but this wasn't
       #a quick refresh on startup
       drop the Bootstrap message silently
     }
   } else {
     drop the Bootstrap message silently
   }

2. Nexthop tracking registration for BSR
3. RPF check for BSR Message.
   Zebra Lookup based rpf check for new BSR
   NHT cache(pnc) based lookup for old BSR

Signed-off-by: Saravanan K <saravanank@vmware.com>