Fix building termcap.db when make obj is run beforehand from a clean tree by
using make variables for the filenames, which helps resolve pathing
appropriately when running cap_mkdb

X-MFC with: r275687
Pointyhat to: me
Fix a bug in r266857: nd6_dad_find() must return NULL if it doesn't find
a matching element in the DAD queue.

Reported by:	Holger Hans Peter Freyther <holger@freyther.de>
MFC after:	3 days