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.
# define VLAN_TPID(hdr, hv)	((((hv)->tp_vlan_tpid != 0) && ((hdr)->tp_status & TP_STATUS_VLAN_TPID_VALID)) ? (hv)->tp_vlan_tpid : ETH_P_8021Q)