disk.cpp: misc changes

- added a "missing" + case ADF_SCP: break; originating from the scp support patch from Keith. **please double-check if this should be included**
- Misc minor compilation warning fixes
- added a check (currprefs.floppyslots[dnum].df != fname) before copying strings, in case they already point to the same address

Does not really make any difference. I don't really care if it is there or not.