selftests: add GRO support to udp bench rx program

And fix a couple of buglets (port option processing,
clean termination on SIGINT). This is preparatory work
for GRO tests.

rfc v2 -> rfc v3:
 - use ETH_MAX_MTU

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>