netlink: Fix C++ compile errors

Allow these files to be included in C++ programs with careful casting to
the proper type, like C++ wants (and in a way that also works for C).

MFC After: 1 week
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1245

netlink: add IPv4/IPv6 attribute writers using in[6]_addr instead of
sockaddrs.

MFC after: 2 weeks

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

carp: support unicast

Allow users to configure the address to send carp messages to. This
allows carp to be used in unicast mode, which is useful in certain
virtual configurations (e.g. AWS, VMWare ESXi, ...)

Reviewed by: melifaro
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D38940

fix buildworld after 595d23f777dc.

Reported by: se

netlink: add basic message writing support to snl(3).

Differential Revision: https://reviews.freebsd.org/D38947
MFC after: 2 weeks

Revert "netlink: make netlink_snl(3) c++ friendly."

Was pushed accidentally.

This reverts commit 629d9219d931e63dc49ef046332b2a360e42a5f6.

netlink: make netlink_snl(3) c++ friendly.

netlink: fix test-includes broken by f2c8381fce9b

Reported by: many

netlink: make snl(3) headers self-contained.

netlink: fix netlink_snl_route.h header includes

netlink: add snl(3) - simple netlink library

Reviewed by: bapt, pauamma
Differential Revision: https://reviews.freebsd.org/D37736