Add raw RX-50 support. These are 400k single sided disks with 80

Add raw RX-50 support. These are 400k single sided disks with 80
tracks and 10 sectors per track. More exotic RX-50 types not
supported, nor is there support for de-interleaving the first two
tracks where the physical sectors are 0 1 2 3 4 5 6 7 8 9, but they
should be interpreted as 0 5 1 6 2 7 3 8 4 9. This is purely to read
the media with dd. The FAT that's on these disks won't work with
msdosfs anyway.