(1)When all related IPv6 addresses are removed,
then remove the associated IPv6 prefix.
(2)When multiple IPv6 link local addrs exist for a same
interface , then let its IPv6 prefix have multiple
interface id, and create multiple IPv6 global addrs with same
interface id.
(3)When a new IPv6 link local addr is assigned for an
interface, then let its IPv6 prefix also have the
interface id of the new IPv6 link local addr, and
create new IPv6 global addrs with same interface id.
Approved by: jkh
eb70be25c7