#3 only refreshes when packet arrives

[Arnout Engelen]

The display is only refreshed when a refresh is due and
a packet arrives.

This appears to be a limitation of libpcap, which
cannot guarantee whether a read timeout will occur when
no packets arrive after a pcap_dispatch().

[Arnout Engelen]

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This can be easily fixed by calling pcap_setnonblock on the
libpcap handle.

Unfortunately, this function is only present in newer
versions of libpcap. I will add a call to pcap_setnonblock
when i can decide which version of libpcap is available
(automake/autoconf, maybe?), or when a newer version of
libpcap is included in Debian stable.