Added a release note.

client/clparse.c
    parse_client_lease_statement () - now calls new discard_duplicate()
    function. This avoids discarding config file leases that match
    lease file leases

client/dhclient.c
    discard_duplicate() - new function that removes a lease from a list
    of leases that matches a given lease, both by address and source

    add_to_tail() - new function that adds a given lease to the end of
    a linked list of leases, removing any duplicates from the list first.

    bind_lease() - if the active lease being replaced by the new
    lease is a fallback lease, then we move it the end of
    client->leases, rather than discard it.

    state_panic() -  now calls add_to_tail() to move the
    active lease that just failed to the end of client->leases.
    This ensures that we keep only the most recent versions of
    dynamic leases.