app/testpmd: display/clear forwarding stats on demand

app/testpmd: display/clear forwarding stats on demand

Add a new "show/clear fwd stats all" command to display fwd and port
statistics on the fly.

To be able to do so, the (testpmd only) rte_port structure can't be used
to maintain any statistics.
Moved the stats dump parts from stop_packet_forwarding() and merge with
fwd_port_stats_display() into fwd_stats_display().
fwd engine statistics are then aggregated into a local per port array.

Signed-off-by: David Marchand <david.marchand@redhat.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>