Add support for stacked VLANs (IEEE 802.1ad, AKA Q-in-Q).

802.1ad interfaces are created with ifconfig using the "vlanproto" parameter.
Eg., the following creates a 802.1Q VLAN (id #42) over a 802.1ad S-VLAN
(id #5) over a physical Ethernet interface (em0).

ifconfig vlan5 create vlandev em0 vlan 5 vlanproto 802.1ad up
ifconfig vlan42 create vlandev vlan5 vlan 42 inet 10.5.42.1/24

VLAN_MTU, VLAN_HWCSUM and VLAN_TSO capabilities should be properly
supported. VLAN_HWTAGGING is only partially supported, as there is
currently no IFCAP_VLAN_* denoting the possibility to set the VLAN
EtherType to anything else than 0x8100 (802.1ad uses 0x88A8).

Submitted by: Olivier Piras
Sponsored by: RG Nets
Differential Revision: https://reviews.freebsd.org/D26436