cache: split negative entry LRU into multiple lists

This splits the ncneg_mtx lock while preserving the hit ratio at least
during buildworld.

Create N dedicated lists for new negative entries.

Entries with at least one hit get promoted to the hot list, where they
get requeued every M hits.

Shrinking demotes one hot entry and performs a round-robin shrinking of
regular lists.

Reviewed by: kib