examples/l3fwd-power: implement proper shutdown

examples/l3fwd-power: implement proper shutdown

Currently, shutdown for l3fwd-power application is all over the place
and may or may not happen either in the signal handler or in the main()
function. Fix this so that the signal handler will only set the exit
variable, thereby allowing all of the loops to end properly and proceed
to deinitialize everything.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Acked-by: David Hunt <david.hunt@intel.com>
Reviewed-by: Reshma Pattan <reshma.pattan@intel.com>