Patching to multiple ROMS/files at once

[DiskPoppy]

Rationale: It's currently impossible to refer to labels in freespace from other patches without writing the label's address to the ROM at known location and then using the known location in another patch (it has also the downside of being easy to mess up when applying the patches in wrong order).

My proposed solution is to be able to write to additional auxiliary file(s) than the one that is being patched, so that patches can communicate and throw a helpful error message when the file doesn't exist when it is being read by another patch. There is already readfile1-4, so only writing needs to be implemented. Maybe even assembling the same way as to the primary ROM with an ability to switch between them.