Add default Xauthority

I have tried guile-wm and got an error.  It happened because i don't have XAUTHORITY environment variable.  I think it would be good to fall back to a default `~/.Xauthority` file in such cases.  (I'm new to guile and am not sure if there is a better way to expand file name).

And thank you for the GREAT work on guile-wm and guile-xcb.