#20 bootsector needs more work to make it self-contained

[Brian Ruthven]

There are some assumptions made within the boot sector. For example, system variable CHARS is assumed to be initialised.

Ideally it would be better to make the boot sector more self-contained, whilst still retaining the ability to produce meaningful errors. Using ROM2 is fine, as there is no explicit knowledge (beyond the published jump table) baked into the bootstrap code, and each routine is designed to be used in isolation if necessary. However, using ROM3 is fraught with potential issues.

Whilst the bootstrap is probably going to be run on a freshly reset system, it would be nice to be able to bzero all RAM then call DOS BOOT, and still successfully boot.