That's my mega-commit you've all been waiting for.

The code for the shell share more routines with userspace apps than with kernel
units, because, well, its behavior is that of a userspace app, not a device
driver.

This created a weird situation with libraries and jump tables. Some routine
belonging to the `kernel/` directory felt weird there.

And then comes `apps/basic`, which will likely share even more code with the
shell. I was seeing myself creating huge jump tables to reuse code from the
shell. It didn't feel right.

Moreover, we'll probably want basic-like apps to optionnally replace the shell.

So here I am with this huge change in the project structure. I didn't test all
recipes on hardware yet, I will do later. I might have broken some...

But now, the structure feels better and the line between what belongs to
`kernel` and what belongs to `apps` feels clearer.

The goal is to avoid mixing those routines with "character devices"
(acia, vpd, kbd) which aren't block devices and have routines that
have different expectations.

This is a first step to fixing #64.

Sure, it's a bit slower, but it prevents a lot of hard to debug
problems. I don't have to want to remember "don't use IX if you
have any blk* calls". Let's optimize I/O later.

Previous refacoring broke all seek/tell within fs. fs handles now lost
the responsibility to keep track of current position. It's blkdev's job.

This huge refactoring remove the Seek and Tell routine from blockdev
implementation requirements and change GetC and PutC's API so that they
take an address to read and write (through HL/DE) at each call.

The "PTR" approach in blockdev implementation was very redundant from
device to device and it made more sense to generalize. It's possible
that future device aren't "random access", but we'll be able to add more
device types later.

Another important change in this commit is that the "blockdev handle" is
now opaque. Previously, consumers of the API would happily call routines
directly from one of the 4 offsets. We can't do that any more. This
makes the API more solid for future improvements.

This change forced me to change a lot of things in fs, but overall,
things are now simpler. No more `FS_PTR`: the "device handle" now holds
the active pointer.

Lots, lots of changes, but it also feels a lot cleaner and solid.

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...

The ability to specify "0" routines in blkdev table is not used anymore
now that stdio is a separate subsystem.

Also, I'm preparing a blockdev refactoring and this complicates my work.

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.

This allows us to break through the 64K limit for includes CFS in zasm,
a limit we were dangerously close to breaking. In fact, this commit
makes us go over that limit. Right in time!

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

scas didn't warn about this, but it seems that `jr` calls were out of
range. Seek and Tell was broken. Replacing `jr` with `jp` does the
trick.

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.

Very coarse, but I got them to work.

This way, we can interact with the mounted FS even when we change the
selected blkdev.

This will facilitate "copying" blk selection in FS.

This allows more than one file to be allocated. `fls` properly lists
multiple files.

Very incomplete, but if you play your cards right, you can, with the
shell:

1. Create a new CFS
2. Mount it
3. Allocate a file with an arbitrary name
4. Have it listed with `fls`

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.

Allows seeking forward, backwards, to the beginning, to the end.

Calling on blkGetC repeatedly was somehow very unstable and often
failed. I didn't pinpoint exactly why, but keeping the call addr around
and calling that instead seems like a better idea anyway.

Instead of waiting, GetC always return immediately, with Z indicating if
something was fetched. The "wait" loop is implemented by the called (and
in the new `blkGetCW`).

This simplifies out-of-bounds verifications for storage blockdevs.

The approach used for shell extra commands also works for blockdev. Much
simpler and tighter this way.

This also introduces the "seek" shell command.

Also, add dummy seek reoutine.

Also, add the ability for parts to define extra shell commands.