namecache: ncnegfactor 16 -> 12

It is used on each new entry addition to decide whether to whack an existing
negative entry in order to prevent a blow out in size, but the parameter was
set years ago and never revisited.

Building with poudriere results in about 400 evictions per second which
unnecessarily grab entries from the hot list.

With the new parameter there are next to no evictions of the sort.