when ipfw isn't configured, the service won't (re)start throwing an error, use...

when ipfw isn't configured, the service won't (re)start throwing an error, use of onestart instead of start fixes this. closes https://github.com/opnsense/core/issues/1292

(cherry picked from commit 5ba5a918)

src/opnsense/service/conf/actions.d/actions_ipfw.conf

 [reload]
-command:/etc/rc.d/ipfw start; /usr/local/etc/rc.ipfw.postload
+command:/etc/rc.d/ipfw onestart; /usr/local/etc/rc.ipfw.postload
 parameters:
 type:script
 message:restarting ipfw