package/iputils: fix license

package/iputils: fix license

- Replace nfinfod/COPYING by LICENSE, Documentation/LICENSE.BSD3 and
  Documentation/LICENSE.GPL2 which are available since version s20180629
  and
  https://github.com/iputils/iputils/commit/686ee13130550aa4cb2937842504b2131f0934c6
- BSD-4-Clause has been removed since version s20180629 and
  https://github.com/iputils/iputils/commit/f69eeac80de413248fd3d7d4d4244af22a81e055

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>