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.
Because the tp_vlan_tpid is always defined as the unsigned (__u16) in TPACKET structures, that change is not needed, IMHO.  And if the definition is changed to the signed in the future, I think this is a kernel's bug...