Floppy writing now works

Writing to floppies now works.

I include a core with 2 drives, so you can see how mounting a disk image sometimes/often(?) results in a garbage directory listing but sometimes unmounting and remounting works. I tend to think this is an M2M problem, not a core problem.

Sound is still missing.

megapet-v0.00003a.zip is a 2-drive core for the Mega-65 R3 board revision.
megapet-1drive-v0.00003a.zip is a 1-drive core for the Mega-65 R3 board revision.