netlink: add NETLINK/NETLINK_SUPPORT userland options.

Make userland tools such as netstat, route, arp and ndp use
either netlink or rtsock interfaces based on the NETLINK_SUPPORT
options.
Both NETLINK and NETLINK_SUPPORT options are turned on by default.

Reviewed By: eugen
Differential Revision: https://reviews.freebsd.org/D39148