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.
Sorry for the noise, I was misreading your points.
The logic for the selecting TPID is same as the TCI, but should it be different logic only in case of the TPID?  It seems pcap-linux uses the value when the kernel gives any value if TP_STATUS_*_VALID is defined.