Update to 1.6
    
    - libndb: reject redirect and router advertisements from non-link-local
    - libndp: validate the IPv6 hop limit
    - libndp: revert API change for ndp_msg_send() and add ndp_msg_send_with_flags()
    - libndp: fix type of field "na" in "struct ndp_msgna"
    - ndptool: add option to send messages types
    - libndp: add option flags to send messages
    - Add SubmittingPatches howto