Add DCAT, DLOAD, DSAVE inspired by Double-Duty DOS

[inexorabletash]

"Double-Duty DOS" by Jason Coleman, in Compute! Magazine, Issue 89 (October 1987) introduces three commands

• DCAT - catalog a DOS 3.3 disk
• DLOAD - load an Applesoft BASIC or Binary file from a DOS 3.3 disk
• DSAVE - save an Applesoft BASIC or Binary file to a DOS 3.3 disk

WIP disassembly at: https://github.com/a2stuff/dddos

Parts like the RWTS code can be dramatically simplified, install/relocation code isn't needed, etc.