netgraph/ng_car: Add color marking code

Chained policing should be able to reuse the classification of
traffic. A new mbuf_tag type is defined to handle gereral QoS
marking. A new subtype is defined to track the color marking.

Reviewed by: manpages (bcr), melifaro, kp
Sponsored by: IKS Service GmbH
Differential Revision: https://reviews.freebsd.org/D22110

(cherry picked from commit d0d2e523bafb74180f8bebb90788790f0d2f0290)