tradindexed expiry speedups

[eagle]

After analysis of a syscall trace of expiration through tradindexed, the following would at least reduce the number of system calls:

• group.index is being opened and closed between every single group expiration, which is really rather unnecessary (and mmap isn't the world's fastest system call, so it may well be noticably slower to do this).

• Overview files that are empty shouldn't be expired in the first place; there's no point, and it would save a bunch of opens and renames.

• Setting the file descriptors to close on exec looks pretty painful in a trace, although I'm not sure how much of a performance hit it really is. It's an additional two system calls for every open, though, and Solaris's system call interface, unlike Linux's, is pretty painful. Those are only in there for innd; we should be able to make this some sort of configuration flag, maybe another flag to pass into OVopen() like the existing modes.