add option to send prefix hint for requesting desired prefix length for delegation

This change adds an option on the interfaces page for sending a prefix hint for the selected delegation size. If enabled, a "prefix" field requesting :: with the appropriate prefix length (64 - dhcp6-ia-pd-len) is added to the "id-assoc pd" entry in the dhcp6c config file. This hint is required for requesting prefixes shorter than /64 from Comcast.