Don't exclude interfaces that aren't up.

On some platforms, you can capture on those interfaces, which might be
useful if you want to start a capture before the interface is up, so
that the capture starts capturing as soon as it's up.

Those interfaces will not appear in the interface list on platforms
where you can't open them for capturing, and they'll now be sorted after
the interfaces that *are* up, so there's not much harm in including
them.
pcap_loop_sample implemented
Updated pcap_loop manpage (added pcap_loop_sample).