This is modeled after VLAN_TAG (which is mislabeled and just returns the
vid instead of the entire tag). I'm ok with changing the behavior of
VLAN_TAG to return the full tag including pcp but I went with a separate
VLAN_PCP to avoid surprising existing users of VLAN_TAG.
I decided to grab the PCP from the ifvlan (like VLAN_TAG does). I believe ifconfig ... vlanpcp <foo> updates this field.
ifconfig ... pcp <foo> updates the if_pcp field but I don't see any code that attempts to keep the ifvlan's pcp
and the ifnet's pcp in sync when the ifnet is a vlan ifnet.