64-bit cleanup

[Yoshinori K. Okuji]

I've checked in a huge patch to support 64-bit disk addressing. I modified a 
lot of code. I also tested it a bit, but I'd appreciate if you can test it 
more.

My change is still not sufficient for filesystems. There are so many 
32-bit/64-bit, signed/unsigned mixtures in filesystem code, so we need to 
refactor the implementations very carefully (in particular, fshelp).

As this part can be improved gradually, I leave this code as it is for now, 
and I will work on something else next.

Okuji