[exim] Feature Requet - Reading Free Memory

I would like the ability to read free memory in ACLs for the purpose of
avoiding overloading the system. Yes - I know I can test load levels but
I'd like to do some preemptive work before memory fills and load levels
get too high. So it would be nice if I knew I was down to the last few
percent of free memory so I could start throwing up some defers and let
the stuff that's being processed pass through before taking in any more
email.

So Phil, next time you're in the mood to add more system stuff, maybe
throw that into the to do eventually pile?

BTW, thanks for doing a great job on this.