bin: added tool mon-ifpps for easy monitoring of PPS stats

Based on one-liner from:
 http://www.spinics.net/lists/netdev/msg293729.html

Quick script that monitors a NICs PPS (Packets Per Sec) rate both TX
 and RX, and also tries to assess the accuracy of the measurements.

For a better/more statistically approach, consider running output
through Rusty Russell's tool: https://github.com/rustyrussell/stats

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