Listen on VLAN using an appropriate BPF filter and VLAN decapsulation.

Listen on VLAN using an appropriate BPF filter and VLAN decapsulation.

Three cases may happen:
 - You have a non VLAN accelerated network card.
   The BPF filter will catch the frames in the VLAN and the frame will
   be decapsulated.
 - You have a VLAN accelerated network card and a kernel < 2.6.27.
   lldpd won't see any frame encapsulated into a VLAN.
 - You have a VLAN accelerated network card and a kernel >= 2.6.27.
   lldpd will see the frames encapsulated into a VLAN as untagged
   frames and won't need to decapsulate them.