pf: Retrieve DSCP value from the IPv6 header

Teach pf to read the DSCP value from the IPv6 header so that we can
match on them.

Reviewed by: donner
MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D29048

(cherry picked from commit f19323847ca894af8a58839f6a2a41691a8e2245)