Fix "grain will be off" by making it not actually off

This fixes the "Suicide Barbie" homebrew thing from dying at startup.  Thanks for the link.

Might be a little dangerous, but I'm pretty sure it's the sane thing to do: downalign the off-grain pointer (that is, allocate the full 0x100 block.)  This way we don't report a free size that is off grain.

Meanwhile, makes it use the same logic for finding the memstick/ directory on Windows as sceIo/etc. does.

-[Unknown]