gluon-mesh-batman-adv: Save if metrics chose neighbor as own best nexthop

The commit ee63ed42fe6c ("gluon-mesh-batman-adv: List neighbors with
non-best direct link") removed the check whether a neighbor has the
BATADV_ATTR_FLAG_BEST set. But consumers may still want to filter out or
mark neighbors which don't have this flag set. To assist with such a
feature, enhance the neighbor object with an extra boolean "best" attribute
which stores whether the BATADV_ATTR_FLAG_BEST was found or not.

Reported-by: Vincent Wiemann <webmaster@codefetch.de>