If we have pcap_breakloop(), at least on UN*X we can stop the capture

If we have pcap_breakloop(), at least on UN*X we can stop the capture
with a pcap_breakloop() call - we don't need to call select() before
calling pcap_dispatch().

Even if we do need to call select(), we don't need to supply it with a
timeout - it's OK if we block indefinitely, as the signal will interrupt
select().

That also means we can pass -1 as the count to pcap_dispatch(), as
pcap_breakloop() will terminate the loop in pcap_dispatch().

Use sigaction() to catch SIGUSR1, so we can make sure that the signal
handler doesn't get reset when the signal is delivered, and that system
calls don't restart when we return from the signal handler.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@18201 f5534014-38df-0310-8fa8-9805f1628bb7