examples/kni: monitor and update link state continually

examples/kni: monitor and update link state continually

Update KNI example to add the command line flag '-m' to enable
a function to continuously monitor the Ethernet link status of
the physical link and update the link status of the corresponding
interfaces with rte_kni_update_link().

Signed-off-by: Dan Gora <dg@adax.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>