[RFA] Make sure target supports non-stop.

[Eli Zaretskii]

> From: Vladimir Prus <vladimir@codesourcery.com>
> Date: Fri, 15 Aug 2008 18:50:47 +0400
> 
> +  if (non_stop && !target_supports_non_stop ())
> +    error (_("Cannot attach to this target in non-stop mode"));

Won't it be more elegant to define a method enter_non_stop_mode for
each target, and have the target complain if it cannot?