Switch netstat -rn to use standard API for retrieving list of routes

Switch netstat -rn to use standard API for retrieving list of routes
instead of peeking inside in-kernel radix via kget.
This permits us to change kernel structures without breaking userland.
Additionally, this change provide more reliable and faster output.

`Refs` and `Use` fields available in IPv4 by default (and via -W
for other families) were removed. `Refs` is radix-specific thing
which is not informative for users. `Use` field value is handy sometimes,
but a) current API does not support it and b) I'm not sure we will
support per-rte pcpu counters in near future.

Old method of retrieving data is still supported (either by defining
NewTree=0 or running netstat with -A). However, Refs/Use fields are
hidden.

Sponsored by:	Yandex LLC
MFC after:	4 weeks
PR:		kern/167204