According to 802.1Q-2014, VLAN tagged packets with VLAN id 0 should be considered as untagged, and only PCP and DEI values from the VLAN tag are meaningful. See for instance https://www.cisco.com/c/en/us/td/docs/switches/connectedgrid/cg-switch-sw-master/software/configuration/guide/vlan0/b_vlan_0.html .
Make it possible to specify PCP value for outgoing packets on an ethernet interface. When PCP is supplied, the tag is appended, VLAN id set to 0, and PCP is filled by the supplied value. The code to do VLAN tag encapsulation is refactored from the if_vlan.c and moved into if_ethersubr.c.
Drivers might have issues with filtering VID 0 packets on receive. This bug should be fixed for each driver.