Like it was already done for 'netstat -i', drop the kvm(3) support
in 'netstat -r'.

The netstat/route.c was the last abuser of struct ifnet and struct
rtentry in the tree. With this change if_var.h can become kernel
only include, _WANT_RTENTRY can go away and projects/ifnet and
projects/routing can go forward.

Differential Revision: https://reviews.freebsd.org/D2242
Reviewed by: melifaro, gnn
Sponsored by: Nginx, Inc.
Sponsored by: Netflix