app/testpmd: added new macfwd-retry mode

app/testpmd: added new macfwd-retry mode

This adds a new forwarding mode to testpmd, whereby it will forward
packets, but on a transmit failure, it will wait a small amount of time
before retrying the transmit again. This mode can be useful in some
cases to set a zero-packet loss rate at the cost of overall throughput,
especially in cases where TX ring size/buffering is constrained, such as
with virtio.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>