IPv6 prefix assignment bug fixes.
IPv6 prefix assignment bug fixes.

    (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

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