FR: DHCPv6 reservations

Add functionality to add DHCPv6 reservations. Given that some networks use DHCPv6-PD, the network id should be optional. The reservation should be tied to a network (vlan).

For example:-

DUID,IAID,VLAN,Prefix,Address
00-01-00-01-15-F9-7F-AB-F0-DE-F1-7A-00-5E, 234890455,4001,,0123:4567:89ab:cdef

This could be complicated by the existence of multiple prefixes on networks, for example a combination of DHCPv6-PD GUA,Static GUA and ULA.