Add an option to prevent cpu stall on slow IO

This is good for any slow storage, including:
  * Hard disk spinning up.
  * Generally slow (cheap) SD cards.
  * HTTP or Samba streaming.

May possibly cause bugs in some cases where timing is unrealistic.  That being said, as long as the game is a UMD game, and there's caching (could enable memory caching for storage), it should not be a problem usually.

This *significantly* improves the gameplay experience when streaming games from HTTP, because you get natural loading screens or short stalls with backgroud music playing (as would happen on a PSP) rather than lag.

I'm also thinking to pull in my io-minor2 changes (potentially making it more accurate now/future) as a "realistic" timing option.

Since fast is probably the best compromise between safe and not having annoying loading screens, I'm not sure there's any good way to avoid an option.  Would appreciate any suggestions on better phrasing.

-[Unknown]
Right, I'll probably mess with it soon.  I had found a bug in sceIo but hadn't really looked into it properly, and forgot:
https://github.com/unknownbrackets/ppsspp/commit/815c0a39af56d43b14087013446b2fad6b7d37fd

Matters a lot more for longer delays... guess this needs it too, really.

-[Unknown]