Fix editing saving dhcp6prefixonly state

Reported in redmine #3097 and forum http://forum.pfsense.org/index.php/topic,64483.msg350255.html#msg350255
Now the config gets the dhcp6prefixonly setting saved. Now that it can be enabled, people who want to use this functionality will need to test it! I don't have a DHCPv6 ISP to test with.
I don't think this little change will help the other issue/s in #3097 when the WAN link is over PPPoE.