sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

Add header definition for RFC4340, Datagram Congestion Control Protocol

Add a header definition for DCCP as defined in RFC4340. This header definition
is required to perform validation when receiving and forwarding DCCP packets.
We do not currently support DCCP.

Reviewed by: gallatin, bz
Approved by: bz (co-mentor)
MFC after: 1 week
MFC with: 350749
Differential Revision: https://reviews.freebsd.org/D21179