Currently, IOCTL only allows the interface-wide adjustment
of the Ethernet 802.1 priority. This adds a new IP_PROTO / IPV6_PROTO
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 to
carry the priority setting