[server] use SIGKILL if necessary for -K

This change sends SIGKILL to fwknopd under -K if SIGTERM does not do the job
first.  This can be necessary in some cases if libpcap does not properly handle
a packet count of zero in pcap_dispatch() (see github issue #110).  On a side
note, the default packet dispatch count of zero will likely be changed because
of that issue too.
[test suite] multi-packet pcap test for pcap_dispatch() validation

This commit adds a new pcap file to the test suite with an SPA packet after
99 other garbage packets.  This can be used for pcap_dispatch() testing,
though this is not meant to be super instensive - it is just to ensure that
if a PCAP_DISPATCH_COUNT of, say, 10 is selected that the SPA is still seen
by fwknopd.  This commit is in support of #110.
[server] pcap_dispatch() packet count default to 100

Updated pcap_dispatch() default packet count from zero to 100.
This change was made to ensure backwards compatibility with older
versions of libpcap per the pcap_dispatch() man page, and also because
some of a report from Les Aker of an unexpected crash on Arch Linux with
libpcap-1.5.1 that is fixed by this change (closes #110).