app/testpmd: add macswap forwarding engine

This commit adds a new forwarding mode, in which the source and destination
MAC addresses of packets are swapped before forwarding according to the
port-topology in effect.  This has been designed for operation primarily with
--port-topology=loopback.

Signed-off-by: Cyril Chemparathy <cchemparathy@tilera.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>