Don't try to rename tmpfiles into existing open files.

Don't try to rename tmpfiles into existing open files.

Linux and friends have no problem with this, but Windows doesn't allow
this without some effort, which we can avoid by... not needing to write
to an already-open file.

Give index.Reader a 'close' method which identifies and closes any open
mmaped files, and make cmd-index.py use this before trying to close a
index.Writer instance (which renames a tmpfile into the same file the
Reader has mmaped).