Some BIOSs initialize the VGA card by default into a mode where the high
bit of background nybble signals that the cell should blink. The simple
way to avoid this is by restricting the background colours to the range
0…7. However, since our mouse cursor is implemented by swapping the
foreground and the background colours, we also need to restrict the
foreground colours to the range 0…7.
Previously programs deallocated their memory before forwarding a
message. If the forwarded message made another program allocate, this
could cause the program code to be overwritten while it is still
qemu deems format autodetection to be dangerous on raw images, so make
sure we specify exactly what we want to do. This will quench a
scary-looking warning right when launching PonyDOS. There's no
functional change otherwise.