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

Up until now, invalid source input had undefined behavior. We're now
beginning to define that behavior so that zasm can be a bit more usable.

To run a parseExpr on first pass would always return a false success
with dummy value because symbols are configured to always succeed on
first pass. This would make expressions like ".fill 0x38-$" so bad
things to labels because "0x38-$" wouldn't return the same thing on
first and second pass.

Revert to parsing literals and symbols after having scanned for
expressions and add a special case specifically for char literals (which
is why we scanned for literals and symbols first in the first place).

Allows for the ".fill x-$" pattern.

When I zasm-ified the shell a couple of dozens of commits ago, I
inverted push and pops...

It was a bad idea to remove it. Now that I'm introducing the concept of
a per-app glue file, it becomes much easier to build emulated zasm as a
userspace app.

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