This would allow things like temporary giving control to the *CL
line on the TRS-80. For example... A very far fetched example. Not
at all the only *raison d'etre* of the layer...

Will be useful for assembling binaries for the TRS-80 which can't start
at addr 0.

I don't remove z80a.fs yet because build process needs it, but that
will change shortly.