trafgen: take advantage of PACKET_QDISC_BYPASS sock option

Since Linux 3.14, the kernel supports a socket option PACKET_QDISC_BYPASS,
which trafgen enables by default. That allow us to bypass the kernels
normal qdisc (traffic control) layer.

An option -q, --qdisc-path is added to allow enabling the qdisc path
explicity, useful for testing purposes.

This will be avail in kernels >= 3.14 via
 commit d346a3fae3 (packet: introduce PACKET_QDISC_BYPASS socket option).

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>