[server] Switched upstart config to use 'expect' section

This change allows fwknopd to write syslog messages to traditional syslog files
while running under upstart.  Not forking into the background resulted in
messages meant for syslog were captured under /var/log/upstart/fwknop.log.

extras/upstart/fwknop.conf

@@ -20,4 +20,6 @@ umask 022
 #     mail -s "Starting fwknop on $HOST" $EMAIL_ADDR < /dev/null > /dev/null 2>&1
 # end script
 
-exec /usr/sbin/fwknopd -f
+expect fork
+
+exec /usr/sbin/fwknopd