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

The rationale is that knowledge of 'struct ifnet' and 'struct rtentry'
by userland tool is on the way of projects/ifnet and projects/routing
respectively.

The usage of netstat as post-mortem debugging tool is questionable. For
example, I have worked on network bugs a lot, but never run netstat on
vmcores. I always used gdb scripts for this purpose.

To advocate more my proposal, I will notice that approximately a year
ago we already did the same change for 'netstat -i', and thanks to
that change, now we got per-CPU counters in struct ifaddr, struct ifnet,
and we got hardware only counters for those interfaces that support
that. Without cleansing netstat/if.c off libkvm(3) that would the former
will be much complicated and the latter simply impossible.