client.c: Added "--nic" option

The client now can be bound to a specific network interface device by using
the "--nic=device_name" ("-n" for short) option, where device_name is the
NIC name (e.g. eth0, wlan0).

Signed-off-by: Erik Alexandre Pucci <eap08@c3sl.ufpr.br>
Acked-by: Ricardo Tavares de Oliveira <rto07@c3sl.ufpr.br>
Signed-off-by: Danilo K. S. Yorinori <danilok@c3sl.ufpr.br>