examples/l2fwd-keepalive: add graceful exit

examples/l2fwd-keepalive: add graceful exit

The l2fwd-keepalive example has infinite processing loops and as a
result the only way to exit it is via SIGINT/SIGTERM (e.g. Control-C).
The resulting shutdown is unclean, which is fixed by adding a signal
handler that causes the processing loops to break.

Fixes: e64833f2273a ("examples/l2fwd-keepalive: add sample application")
Signed-off-by: Remy Horton <remy.horton@intel.com>
Tested-by: Roman Korynkevych <romanx.korynkevych@intel.com>