Also, add a port for rewinding stdin.

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.

I realize that libz80 is GPL and that this project is MIT. That's not
much of a problem as long as libz80 isn't a big part of the project, but
I'm planning on building more around libz80 so that zasm can replace
scas as an assembler even on modern machines (through an emulated
layer) so that this project can become entirely self-bootstrapped.

If I'm going to do that, I might as well be kosher license-wise. I
don't care much about whether the project is GPL or MIT...

Also, adjust SD card recipe. Straightforward initialization and read!

Makes for a more straightforward test.

Incomplete, but what it does *does* work. I could handshake CMD0+CMD8 on
a MicroSD HC.

Without delays between read/writes, I often add lockups.

It always reads in the scratchpad with the same max size. No need for
DE-dancing anymore.

De-couple "success" from "wrote something to (DE)" so that directive
parsing can be considered successful without writing to (DE).

It will make tokenization cleaner and it also makes the instruction
table significantly more compact.

instr.asm is for single instruction assembly.

* Build emulated zasm statically
* Improve comments in zasm.asm
* Fix build
* Use unsetZ from core

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.