one more patch for ovdb

[Heath Kehoe]

I have just mailed to inn-patches a patch for ovdb.c that needs to
go into 2.3 before its release.  (also should go into 2.4).

It (probably) won't fix the problems that some people here have
been having.

I think a note should be added to the 2.3 documentation that says
something like: "ovdb has not been as widely tested as the other
overview methods, and should be considered 'experimental'."

-heath