Check memory stick free space and tell the game about it

This isn't necessarily super well tested, in that I did not fill up my disk and see if it made any games freak out in incorrect/correct ways.  However, I did verify that this doesn't make games freak out (at least the ones I tested) when you *do* have free space.

This also changes the memory stick (except in old savestates) to be 8GB.  Sony did sell an 8GB stick, so that shouldn't be an unusual size for games to see.  If you have more than 8GB free, it always just says 8GB.  In practice, I suspect 32GB or higher might work fine (as I have 32GB on my PSP, actually, and have never had a problem.)  Incidentally, I've never run out of space on my PSP (although I did on my Android device which actually had less space.)

This change may affect behavior of some games.  For example, Dissidia refused to export highest quality avis with the previous setting iirc.  It's possible it may trigger bugs or forced installations that didn't happen before, in the worst case.

On Symbian, it always lies and says there are 8GB free, since I don't know how to detect disk space on that platform.

This improves #6593.  However, surely there are still error codes and such we ought to be providing in that situation, which is probably what most games react to if anything.

-[Unknown]