Clean up per-packet output a bit; don't output a blank line if no

Clean up per-packet output a bit; don't output a blank line if no
packets were seen, but do put the "."s on a separate line - which should
all come out in a burst, so don't fflush() after each one, let the
newline at the end do that.

Report POLLNVAL for poll().

When not using select() or poll(), loop forever doing pcap_dispatch(),
rather than just using pcap_loop(), so you see what happens for each
delivered batch of packets.