The catch is that during configuration file processing,
no cache is open (yet), as kresd can't know if the config
does open it in some later part (with non-default path or size).
Now we just throw an error.  Exceptions:
 - cache.open() and cache.backends(), of course :-)
 - cache.ns_tout() - not required, it's not really inside cache
 - cache.close() - it sounds reasonable to allow "closing a closed cache"

This immediately caught a typo in cache metatable.