overview index cache

[Katsuhiro Kondou]

Seeing from my server in production, not a few reader issue
HEAD and BODY to retrieve articles.  Current article retrieval
in nnrpd calls OVgetartinfo() for each article to get token.
I believe it's quite wasteful for some overview method(yes,
I mean buffindexed).  I'm thinking of having overivew index
cache to reduce this, but some methods may not care those
behavior.  All methods may not need.  To match both I'll add
new feature into OVctl() which will do something like madvise(),
to inform method to keep or free cache for index.  For method
which does not care, just returns with doing nothing.  Any
comments?
-- 
Katsuhiro Kondou