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