examples/l2fwd-event: add graceful teardown
examples/l2fwd-event: add graceful teardown

Add graceful teardown that addresses both event mode and poll mode.

Signed-off-by: Pavan Nikhilesh <8184a94f8f7778086ea0697cdb513b2e42813b09@marvell.com>
Acked-by: Nipun Gupta <4871fd67344c57328097f2fd932ad9a0f1f1fe74@nxp.com>
Acked-by: Jerin Jacob <352c4d4f9291b869992ddb67daa45ddc149fed30@marvell.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk