new: usr: Add 6to4-self style rules for /64 to /48 nibble boundaries

Added self update-policy rules for reverse names for IPv6 prefix lengths /48, /52, /56, /60 and /64 being common ISP prefix delegation size. The rule names are 48-self through 64-self and behave similarly to 6to4-self.

A typical use would be where you have an IPv6 prefix delegation pool and this would allow the clients to delegate reverse zones matching the prefix delegation.

Prefix delegation pool: 2001:DB8::/32 with /48 delegations.

zone 8.b.d.0.1.0.0.2.IP6.ARPA {
    ...
    update-policy {
        grant * 48-self . NS(10) DS(8);
        grant dhcp6-server-key subzone ANY;
    };
};

This allows secure delegations to be added to 8.b.d.0.1.0.0.2.IP6.ARPA at 0.0.0.0.8.b.d.0.1.0.0.2.IP6.ARPA through f.f.f.f.8.b.d.0.1.0.0.2.IP6.ARPA and for the DHCPv6 server to remove the delegation when the prefix delegation expires.

Includes #4751 (!9074)

Closes #4752