rainbow.c: New floppy code - uses more reliable 'wd_fdc'. [Bavarese]

i86.c: Four new 8086 opcode aliases: 0xc0 is 0xc2; 0xc1 = 0xc3; 0xc8 = 0xca; 0xc9 = 0xcb as documented at OS 2 Museum. [Bavarese]

rx50_dsk.h/c: Rainbow specific; The RX50 _disk drive_ was used in different architectures.  Floppy format used by Rainbow-100 and -190. Track layout (post-amble, some positions) yet unverified. Currently only raw 400 K disk dumps are supported (*.img).  [Bavarese]