netgraph/ng_vlan_rotate: IEEE 802.1ad VLAN manipulation netgraph type

This node is part of an A10-NSP (L2-BSA) development.

Carrier networks tend to stack three or more tags for internal
purposes and therefore hiding the service tags deep inside of the
stack. When decomposing such an access network frame, the processing
order is typically reversed: First distinguish by service, than by
other means.

This new netgragh node allows to bring the relevant VLAN in front (to
the out-most position). This way other netgraph nodes (like ng_vlan)
can operate on this specific type.

Reviewed by: manpages (gbe), brueffer (manpages), kp
Relnotes: yes
Sponsored by: IKS Service GmbH
Differential Revision: https://reviews.freebsd.org/D22076

(cherry picked from commit cfd6422a5217410fbd66f7a7a8a64d9d85e61229)