Interfaces: Other Types: VLAN - Add stacked VLAN support (IEEE 802.1ad / QinQ)

[AdSchellevis]

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Is your feature request related to a problem? Please describe.

Since FreeBSD 13 its possible to stack vlans on top of vlans (IEEE 802.1ad), although in the past it should have been possible to build a QinQ using netgraph, it didn't work reliable forcing us to remove it back in 2018 (#2274).

Given the new functionality it seems like a good idea to give this another try as the interface is much cleaner now.

Describe the solution you like

Ideally the possibility to select a vlan in the vlan configuration, maybe refactor the current frontend while there.

Additional context

• https://reviews.freebsd.org/D26436

[mimugmail]

Setting a Vlan as parent interface might also make sense from a user perspective