net: Introduce IPV6_DSCP(), IPV6_ECN() and IPV6_TRAFFIC_CLASS() macros

Introduce convenience macros to retrieve the DSCP, ECN or traffic class
bits from an IPv6 header.

Use them where appropriate.

Reviewed by: ae (previous version), rscheff, tuexen, rgrimes
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D29056

(cherry picked from commit bb4a7d94b99fbf7f59c876ffff8ded5f6a5b5c3e)