route(8): convert to netlink

This change converts all kernel rtsock interactions in route(8)
to Netlink.

Based on the WITHOUT_NETLINK_SUPPORT src.conf(5) variable, route(8)
now fully operates either via Netlink or via rtsock/sysctl.
The default (compile-time) is Netlink.

The output for route delete/add/get/flush is targeted to be exactly
the same (apart from some error handling cases).
The output for the route monitor has been changed to improve
readability and support netlink models.

Other behaviour changes:

• exact prefix lookup (route -n get a.b.c.d/e) is not yet supported.
• route monitor does not show the change originator yet.

Differential Revision: https://reviews.freebsd.org/D39007