Avoiding the OOM killer with mem_notify

I don't think I've ever written a substantial program that didn't have 
*some* sort of caching in it, to trade off space against time somewhere 
where `spend the time, every time' was undesirable. Often these caches are 
not expected to be terribly large, and have the exciting expiration policy 
`never', but it would be fairly trivial to respond to a mem_notify signal 
by just ditching the entire contents of all of those caches.