OSPF: Add support to multi-area to Router Info.

OSPF: Add support to multi-area to Router Info.

  Router Information needs to specify the area ID when flooding scope is set to
  AREA. However, this authorize only one AREA. Thus, Area Border Router (ABR) are
  unable to flood Router Information Opaque LSA in all areas they are belongs to.

  The path implies that the area ID is no more necessary for the command
  'router-info area'. It remains suported for compatibility, but mark as
  deprecated. Documentation has been updated accordingly.

Signed-off-by: Olivier Dugeon <olivier.dugeon@orange.com>