examples/l3fwd-power: add legacy mode option
examples/l3fwd-power: add legacy mode option

Currently, legacy mode is the implicit default, but it is not possible
to directly request using legacy mode. Add the argument to enable
requesting legacy mode, and also make it the default.

Signed-off-by: Anatoly Burakov <814e00d8f6e4e81b7ccc7e3e5a0bfec0078f0203@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk