Disable _revers handling.

This doesn't work as expected because the CPC reverse swaps the colors,
rather than inverting the characters.
Remove useless trace.
Make DSC files relocatable.

* Make the header const. It will be modified after loading, but making
it non-const would need a gsinit, wasting disk space and RAM.
Implement cclearxy using SCR_FILL_BOX

This is a lot faster than printing chars one at a time.
Optimizations...

* Make the CRT0 even smaller. The gsinit code can be overriden, too.
* Remove bitmap icons as we currently don't use them
Hardcode AMSDOS RSX address for now.

The cpde to get it at runtime doesn't seem to work.