Register cache

[Andrew Cagney]

> Is this really a good idea?  I think it is, at least on targets where
> getting at the registers is relatively expensive since it involves a
> system call.  Therefore I propose to make this "official" and change
> the comment on target_fetch_registers() to:
> 
>    /* Fetch at least register REGNO, or all regs if REGNO == -1.  */

This behavour is pretty much engrained in GDB so yes, the documentation
should be changed to reflect this.

> Another question is whether we should make this behaviour (I mean
> having target_fetch_registers() supply all registers it managed to get
> in one go) recommended practice.

I'd agree with Nick and not go that far.  Keep the contract between
GDB's core and the target simple.

	Andrew