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.

My mind isn't so good at decimal -> hex translations. It helps to have
this...

This way, we can compile apps from within the shell that run on the
shell.

What used to be `tools/emul/user.h` was in fact specific to zasm, so I
moved it there.

To avoid name confusion, I renamed what used to be kernel.h and user.h
to kernel-bin.h and user-bin.h.

This makes for a much more usable zasm linux binary that doesn't have to
be rebuilt every time apps or kernel change.

The next step is to generate them on-the-fly.

Now that we don't sed out the comments any more, we busted the buffer.

... instead of embedding it in the binary itself. Additionally, add a
"zasm.sh" wrapper to faciliate zasm calls on a linux machine.

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.

Wriiiiiite!

That took an embarrasingly large amount of time to debug for something,
in the end, so obvious...

The pgm module implements a shell hook so that when an unknown command
is typed, we look into the mounted filesystem and look for a file with
the same name as the command. If we find one, we load it in memory and
run it.

Using HL/DE was awkward and error-prone.

It would previously only work when GetC-ing our way into a new sector.
Seeking into one would not work. Now it's much more robust and this
paves the way for write support.

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

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!

For now, top-level only