[RFA] Add set/show debug unwind command

[Joel Brobecker]

> 2012-06-15  Tristan Gingold  <gingold@adacore.com>
> 
> 	* frame-unwind.c (show_unwind_debug): New function.
> 	(unwind_debug): New variable.
> 	(_initialize_frame_unwind): Add show debug unwind command.
> 	* frame-unwind.h (unwind_debug): Declare.
> 	* doc/gdb.texinfo (Debugging Output): Document set/show debug
> 	unwind.
> 	* NEWS: Mention it.

Looks good for the code part, just one tiny comment:

> +  /* Debug this files internals.  */
                   file's

-- 
Joel