pcap-linux: Obtain VLAN TPID from the kernel when available

The VLAN TPID was hardcoded in pcap-linux.c but recent Linux kernel
supports 802.1ad Q-in-Q.
Since Linux 3.14, kernel gives the TPID value to the userspace when
available.
The reason I posted question was that second TP_STATUS_VLAN_TPID_VALID does nothing meaningful when conbined with series of past linux kernels. If a packet has tpid, then first condition will be met, because tpid won't be 0.

Anyway, vlan tag existence will be checked by vlan_tci, and not vlan_tpid. If TP_STATUS_VLAN_TPID_VALID is defined, then we need just check that bit without checking tp_vlan_tpid is 0 or not, IMHO.