* Document _write

 * Add ability to wait for debugger to attach

Cleanup some comments.

Add ability to use vendor commands with programmers.

* Blink now blinks D4 too
* Don't use compressed instructions by default.  Some compilers are old.
* Prevent double-halting on linke programmer.

Update build instructions on Windows to show how to use the off-the-internet build chains instead of WSL2.

Also fix 2 warnings.

 * Fix sandbox compiling.

Switch over to using a .c file instead of a .S file for the startup code.  Helps make order-of-compiling not matter.