It would always load one byte less than told... This time it's right on
the money :)

Calling `load` would always read one more byte than specified. Also,
make `0` mean `0x100`.

I'm about to change that structure and I don't want fs to be messed up
by this. I'm proceeding step by step...

This allows us to reliably and easily load a file until the end of it.

Move load/save to blkdev_cmds and add a new "poke" builtin shell cmd
that is the mirror of "peek" and strictly uses stdio (no blkdev
involved).

This allows us to slim the minimal OS size but, more importantly, change
the behavior of "load" so that we don't expect GetC to block until Z is
set. This way, using "load X" with X being larger than the blkdev size
won't block forever.

This also brings our RC2014 minimal kernel below the 1K mark again.

So that blkdev not supporting 32-bit aren't messed up.

Let go of that "meta os" thing. it's not as meta as I made it sound
like. It's a kernel.

This will soon allow use to seek and tell on input, which is necessary
for a second pass which is necessary for forward symbol references.

This require making `blkSel` a bit more flexible. Rather than having one
global selected blkdev, each app can select its own, in its own memory
space.

Also, clarify the role of recipes.

It wasn't a good idea to complicate all blockdev impls with complicated
seeks. Let's just stay in absolute mode for now.

Also, extract commands into blockdev_cmds to avoid making blockdev
dependent on stdio.