spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

fix buildworld after 595d23f777dc.

Reported by: se

netlink: add rtsock-compatible header to use with netlink snl(3).

Some routing socket defines (`RTM_` and `RTA_` ones) clash with the ones
used by the the Netlink.
As some rtsock definitions like interface flags or route flags are used in
both netlink and rtsock, provide a convenient way to include those without
running into the define collision.

Differential Revision: https://reviews.freebsd.org/D38982
MFC after: 2 weeks