Add IP(V6)_VLAN_PCP to set 802.1 priority per-flow.

This adds a new IP_PROTO / IPV6_PROTO setsockopt (getsockopt)
option IP(V6)_VLAN_PCP, which can be set to -1 (interface
default), or explicitly to any priority between 0 and 7.

Note that for untagged traffic, explicitly adding a
priority will insert a special 801.1Q vlan header with
vlan ID = 0 to carry the priority setting

Reviewed by: gallatin, rrs
MFC after: 2 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D26409