By default, fwknopd will now exit if the interface that it is
sniffing goes down (patch contributed by Github user 'sgh7'). If this
happens, it is expected that the native process monitoring feature in
things like systemd or upstart will restart fwknopd. However, if fwknopd
is not being monitored by systemd, upstart, or anything else, this
behavior can be disabled with the EXIT_AT_INTF_DOWN variable in the
fwknopd.conf file. If disabled, fwknopd will try to recover when a
downed interface comes back up.