allocate memory for pcap files on the socket of the (first) tx core/port

Pktgen fails to load pcap files when started with --socket-mem 0,4096
(no memory allocated on socket 0), because it always tries to create
the mempool on socket 0. This patch changes that to allocate the
mempool on the socket of the first tx core for each port.
Merge pull request #13 from zendes/mempool_alloc_fix

allocate memory for pcap files on the socket of the (first) tx core/port