cache: decrease ncnegfactor to 5

The current mechanism is bogus in several ways:

• the limit is a percentage of total entries added, which means negative

entries get evicted all the time even if there are plenty of resources

• evicting code is almost not concurrent, which makes it unable to

remove entries fast enough when doing something as simple as -j 104
buildworld

• there is no support for performing mass removal if necessary

Vast majority of negative entries never get any hits. Only evicting
them when the filesystem demands it results in a significant growth of
the namecache with almost no improvement in the hit ratio.

Sample result about afer 90 minutes of poudriere -j 104:

current    no evict   % of the original

numneg 219737 2013157 916
numneghits 266711906 263544562 98 [1]

[1] this may look funny but there is a certain dose of variation to the
build

The number was chosen as something which mostly eliminates spurious
evictions during lighter workloads but still keeps the total at bay.

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