[MLton] totalSwap info in runtime/gc.c

[Stephen Weeks]

> I think I will try to overshoot - I personally never close 
> applications, so I use swap heavily for all those currently unused 
> apps, don't want applications to break because of this :)

To be clear, the totalRam, at least as we're using it on all our other
platforms, is *not* the available RAM and does not take into account
other apps.  It is the total amount of RAM in the machine.

It would be nice if MLton played better with other apps, and the
runtime payed attention to how much paging was going on, but I've
never found time or enough motivation to work on this.

> Swap + half of the available disk space where the swap is, should be a 
> good approximation, but I am open for better ideas.

Sounds fine to me.