[Coco] Update for RGBDOS ROMs to correct an obscure disk I/O error

[Robert Gault]

I was notified by Louis Fernández and Bill Pierce that VCC does not recover from 
disk I/O errors when RGBDOS is used. I've found that this was caused by some 
missing code in the RGBDOS ROM.
While the bug will not be seen for the most part unless trying to read unmounted 
disks, it is annoying and needs correction.

I have a new version of RGBDOS for the JVC Coco3, MESS, and VCC emulators. In 
addition to correcting the bug, there is now support for two .vhd images mounted 
at the same time. This is possible in the JVC John Collyer Coco3 and recent 
versions of MESS.

I hope to get the updates posted to my web page on Aaron Wolfe's site shortly.

Robert