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.
Yes, that's right.  But then I think the macro just should be as following:
```
# define VLAN_TPID(hdr, hv)	((hdr)->tp_status & TP_STATUS_VLAN_TPID_VALID ? (hv)->tp_vlan_tpid : ETH_P_8021Q)
```
Of course the tp_vlan_tpid will be used in this case even if the TPID is 0, but it ought to be right in the userspace, because the kernel set the TP_STATUS_VLAN_TPID_VALID.