X11 support

[Roland Illig]

Roland Illig wrote:
> Hi all,
> 
> I have written a module to get the X11/GModule support out of src/key.c. 
> Separate things---separate files.
> 
> With this module src/key.c can be simplified. I also added the framework 
> for defining a custom error handler. This should be a good step on the 
> way to eliminate the "openssh bug".

Does anyone have objections against this code? Otherwise I'll commit it.

Roland