Option to not use Z80 shadow registers / undocumented Z80 instructions / all non-8080 instructions

[Kroc]

Some CP/M systems, and particularly Zeal 8-bit OS, use the shadow registers so we cannot guarantee their safe use.
This is not the same as 8080-support, as that would also require not using the IX/IY registers and whatever else the Z80 added.

This will not provided for the v0.x branch as the WLA-DX version of v80 is only ever intended to run on NTVCM / RunCPM.