Add support for VLAN protocol
Introduce a vlan.protocol property that specifies the protocol of a VLAN, which controls the tag (EtherType) used for encapsulation.
Regular VLANs use 802.1Q (tag 0x8100). To implement VLAN stacking it's sometimes useful to have 802.1ad VLANs with tag 0x88A8.
The property is a string instead of e.g. an enum because this allows maximum flexibility in the future. For example, it becomes possible to specify an arbitrary number in case if the kernel ever allows it.