[Simh] Printing and parsing registers

[Bob Supnik]

When flag REG_VMIO is set, printing/parsing are done through the 
user-supplied print/parse routines, a special switch is set, and the 
normal UNIT pointer argument is left as null.

For the REG_VMIO case ONLY, scp could fill in the UNIT pointer argument 
with a pointer to the register data structure (cast to pointer to unit), 
which would allow fprint/fparse to know what register is being 
referenced and to pick up any additional information that might be 
necessary for printing and parsing, including flags added to their 
proper spot, the flags field.

The problem, of course, is that (REG *(UNIT *(reg structure)) may not be 
idempotent on some hosts (like word-oriented machines), not that many of 
these exist anymore. However, I think it's cleaner, in general, to put 
flags where they belong and find a way to let the print/parse routines 
know what register they're working on if REG_VMIO is set.

/Bob