netstat: Make netstat -rn more resilient to having the routing table changing

netstat: Make netstat -rn more resilient to having the routing table changing

Obtained-from: FreeBSD 176289 by jhb
  "Make netstat -rn more resilient to having the routing table change out from
   under it while running.  Note that this is still not perfect:
   - Try to do something intelligent if kvm_read() fails to read a routing
     table structure such as an rtentry, radix_node, or ifnet.
   - Don't follow left and right node pointers in radix_nodes unless
     RNF_ACTIVE is set in rn_flags.  This avoids walking through freed
     radix_nodes."