ospf6d: fix argument processing in the "area ... range" command

ospf6d: fix argument processing in the "area ... range" command

* When the "cost" argument isn't present, the default cost should be
  used instead of preserving the previously configured one (if any);
* When the "not-advertise" argument isn't present, the "not-advertise"
  flag should be unset regardless if it was previously configured or
  not.

Configuration commands should be deterministic and work in the same
way regardless of the current state.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>