fix TPACKET_V3 initialization (GH #329)

libpcap would fail to initialize on Linux when compiled against kernel
headers with TPACKET_V3 support but running on kernel without:

"can't get TPACKET_V3 header len on packet socket: Invalid argument"

Fix init_tpacket() to respect EINVAL because that is what the kernel
returns for unsupported TPACKET versions.