Teach services code about start stop restart of dhcrelay6

See forum: http://forum.pfsense.org/index.php/topic,64399.0.html
Now:
a) When enable DHCPv6 Relay is unchecked, the service process is stopped in real-time (as well as saving the "disabled" status in the config).
b) DHCPv6 Relay is seen in Status-Services and the dashboard services widget.
c) DHCPv6 Relay can be stopped, started, or restarted from the displays at (b).