netlink: add NETLINK to GENERIC.

This is a followup of 692e19cf5195 (add netlink to GENERIC@amd64).

Netlink is a communication protocol defined in RFC 3549. It is async,
TLV-based protocol, providing 1-1 and 1-many communications between kernel
and userland. Netlink is currently used in Linux kernel to modify, read and
subscribe for nearly all networking states. Interface state, addresses, routes,
firewall, rules, fibs, etc, are controlled via Netlink.

Netlink support was added in D36002. It has got a number of improvements and
first customers since then:

• net/bird2 got netlink support, enabling route multipath in FreeBSD
• netlink-based devd notifications are being worked on ( D37574 ).
• linux(4) fully supports and depends on Netlink

Enabling Netlink in GENERIC targets two goals.
The first one is to provide stability for the third-party userland applications,
so they can rely on the fact that netlink always exists since 14.0 and potentially 13.2.
Loadable module makes life of the app delepers harder. For example, net/bird2 can be
either build with netlink or rtsock support, but not both.

The second goal is to enable gradual conversion of the base userland tools
to use netlink(4) interfaces. Converting tools like netstat (D36529), route,
ifconfig one-by-one simplifies testing and addressing the feedback.
Othewise, switching all base to use netlink at once may be too big of a leap.

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D37783