read_register_byte can't work with pseudo-reg model

[Andrew Cagney]

> (something else on my todo list is stomp on some of these redundant 
>> interfaces :-)
>> 
> 
> 
> 
> Hmm, how about doing some of this stomping in a more aggressive manner on 
> the regbuf branch; causing temporary breakage for some targets should be 
> less of an issue there.

I prefer the technique of adding legacy_ prefixes and then deleting 
``legacy'' targets :-^

More seriously, I've re-done the reg stuff to have a regcache (like you 
suggested) (yes we both fixed the same param bug) instead of a regbuf - 
regbuf was overkill for the problem immediatly to hand.

Part of this change will involve replacing two interfaces.  Getting just 
that right will be fun enough.

enjoy,
Andrew