lldp: Read VLAN

Some devices (notably Cisco IOS) send frames on tagged with the native
VLAN while they should send them untagged. If your network card does
not support accelerated VLAN, you will receive those frames as long as
the corresponding interface exists (see below). However, if your
network card handles VLAN encapsulation/decapsulation (check with
`ethtool -k`), you need a recent kernel to be able to receive those
frames without listening on all available VLAN. Starting from Linux
2.6.27, lldpd is able to capture VLAN frames when VLAN acceleration is
supported by the network card. Here is the patch:

 * http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=bc1d0411b804ad190cdadabac48a10067f17b9e6