src/oping.c: Added -D <interface name> command line option.

src/oping.c: Added -D <interface name> command line option.

This option sets the outgoing network device to use.

I decided not to reuse the -I command line option for that purpose (which is
what GNU ping is doing), for the following reasons:

 * In theory, an interface name could be confused with a host name.
 * Imho, it's better to specify things explicitly.
 * This approach is more flexible.